The Angry Video Game Nerd

Our main character and the base for any comparison. He can shoot in eight directions and shoot from staircases and has no cap to the ammount of shots he can have in screen. He is the only one that can use the Superscope powerup which improves his damage until he takes a single hit. His jump and speed are considered medium by this walkthrough.
You can go through the whole of the game with just him, including the last boss but not the cameos. Some areas might be more problematic though.
His Light Phaser gun changes color depending on the difficulty you're in.

The Guitar Guy

The first extra character. He is faster but can jump a whole block height less. His shot can pass through walls and many entities as well but he has a cap on the ammount of shots he can get per second.
He's pretty useful for tight areas where you have to speed through deathtraps and for the flying segments where he can wave shot a huge area while moving fast enough to avoid obstacles.

Mike

Guitar Guy is needed to get Mike. Mike is slower than the Nerd but can jump a little higher, comparably to Luigi from Mario 2. He uses a lightsaber to attack and cannot move while doing so. Most of the enemies aren't really vulnerable to it, but slow and ground-based enemies like the octoroks and slimes can be one-hit by him. He can see invisible platforms and breakable blocks and interact with them.

♥♥♥♥♥♥♥♥ Man

Aka B.S. Man, obtainable by using Mike in a certain stage. You know what's ♥♥♥♥♥♥♥♥!? The censorship in these guides, that's ♥♥♥♥♥♥♥♥! He is the slowest and jumps lower than everyone else, but he can double jump, which makes a few jumps only possible to him. He throws some ♥♥♥♥♥♥ hadouken that arcs and causes a lot of damage, but might be hard to use because of its delay and arc.

To get all characters as fast as possible you should go to Future ♥♥♥♥♥♥♥♥♥ 2010, Dungeons & ♥♥♥♥♥♥♥♥♥ and Beat It And Eat It in this order.
You don't actually get any powerups of any sort from completing the stages so you might do any other stage in any order, but having all characters might make some of them easier.

Just before the end of the level you can kill Naggi for your first achievement. You can't redo this stage without restarting the game, so get that gun on.

After the falling spikes you can check the blocks below with Mike for beer and a life.
By the next checkpoint you can break the wall to get a height advantage on the miniboss.
On the next one you can use ♥♥♥♥♥♥♥♥ Man or Guitar Guy for the miniboss.
After that there area staircases like castlevania's with invisible platforms Mike can use to find a life, two beers and the cameo Brent.
Go left afterwards for a life.
You can make the jump left of this with ♥♥♥♥♥♥♥♥ Man for two lives, a beer and another cameo.
In the moving platform area you can break a wall with Mike for possibly a random occurring item.
Two checkpoints later there's the instadeath labyrinth. There'll be a life by your left, and a breakable wall below the skull blocks that'll be a shortcut and have a life inside.
The longer areas are easier to walk through with Guitar Guy for sheer speed. Get a life at the end of it. When playing solo with the Nerd one of the instadeath platforms was actually a fake floor, but I haven't been able to redo it with another character in the party.
Checkpoint. Another life at the left, you can use Mike for an easier time collecting it.
This miniboss is at the perfect range for ♥♥♥♥♥♥♥♥ Man.
When you reach the bottom, go all the way to the left for ♥♥♥♥♥♥♥♥♥♥.
You might want to ignore the Superscope and use Guitar Guy and Mike for this next part. The miniboss is in range for Guitar Guy.
The boss, Jekyll and Hyde. Avoid his fireballs until he becomes Jekyll and use the Nerd to mash half his health in a cycle. His cycle is pretty straightforward and dependant on your position, use that to your advantage.

At the first life item, ignore it. You might be able to get it at Easy where it's useless, but it's a deathtrap in any other difficulty.
Next checkpoint go right for cameo from Back To The Future and two lives, a beer, Super Mecha Jesus and four Superscopes.
During the ascent there's a life on the left of the next checkpoint, it's pretty safe.
Left to the next checkpoint is ♥♥♥♥♥♥♥♥♥♥.
You can fall down the stairs to the left to find the Keith Cameo, three lives and four beers.
There's a Glitch in the way, but it's not worth it.
During the spike corridor you can jump the period the spikes are up.
Next checkpoint, better to avoid the purple testicles than to fight them.
Above the next checkpoint you can jump on the wall with Mike and get a life as well as skip the spikes.
For the boss, get Guitar Guy above the flying platforms, just in the middle, crouch, and shoot away. That's all.

At the start in the lasers there is a hole between platforms with a jellyfish in it, you can jump directly to the next height with the Nerd's jump.
Just after this to the right there is a staircase, the single block to its right is breakable by Mike. This is useless, however.
After the next checkpoint there's a pit and a floating life. You can get there with Mike by seeing the invisible platforms. ♥♥♥♥♥♥♥♥♥♥ will be to your right.
The next stairs are above breakable blocks. There's a Super Mecha Jesus inside.
The next checkpoint is by the appearing and disappearing platforms. There are four vertical lasers to the right. Go that way to unlock Guitar Guy and get an extra beer and life.
Five checkpoints later there's a life just to your right.
Next checkpoint starts the silver surfe- I mean surfboard stage.
Next is the boss. Get the Nerd up front and mash that trigger until she falls; You have to hit her 'children' first and then she'll go ballistic. Avoiding her shots might be harder than just mashing.

Use ♥♥♥♥♥♥♥♥ Man for the first miniboss from above the stairs.
You can use the Nerd for the second one, mash those buttons. There's a life on the right, it's pretty safe.
Next checkpoint go up and left and take Mike out for invisible platforms leading to the cameo Egoraptor, two lives and two beers. Then just go right and break the walls. You can either keep going right or go down and break another wall.
The flamethrowers are a problem but you can use ♥♥♥♥♥♥♥♥ Man to get slower movement and slides from it.
Next checkpoint you can fall down and use the Nerd to mash the balls and the miniboss. The miniboss has a life within.
Just right of the miniboss there's a jump you can make with ♥♥♥♥♥♥♥♥ Man. You can find the cameo Joe, three lives and two beers.
There's a Glitch below the turrets but it'll cost you some backtracking to return.
There's a life between instadeath blocks and a flamethrower. I'd rather not try.
Next checkpoint go with Mike over the lifts and go left for invisible platforms, two lives and ♥♥♥♥♥♥♥♥♥♥.
Next checkpoint, be careful with the lifts and the spikes in the ceiling. You might have a better time with Guitar Guy, just don't jump on the last two lifts.
Santa... Mash up with the Nerd.
This next sequence is about memorization. You can shift characters to change your overall speed. The second floating platform can give you a decent jump. Hold forward on the jump while having Nerd for a nice skip.
This boss might be a problem. Avoid the ghosts of christmas while they spin and then mash with the nerd when they group up. You can try to stay in the center and avoid the ice, but it's also easy to fall down and die.

The first pit can be a shortcut. Just get the Nerd and fall to the left.
Next checkpoint: To the right there is a tall instadeath block. Mike can jump over it, although it is kinda tricky. Bossman Craig and another cameo are there and you also skip a part of the stage. There are two beers and three lives in there as well.
You can backtrack from the checkpoint for a life.
Kill the shark and all aboard. Control Guitar Guy for this area and shoot nonstop.
Next checkpoint has a Superscope, your choice.
Two checkpoints later go right for a life and ♥♥♥♥♥♥♥♥♥♥.
There's a life below the staircase, you can jump back up with Mike or the Nerd. It can be risky because of the instakill block.
Against the swarm of enemies you can use Guitar Guy for an easier fight. After those, the slime monsters can be one-hit by Mike.
The boss can be done in by the Nerd by locking your angles, by the Guitar Guy or by ♥♥♥♥♥♥♥♥ Man for more damage each hit. Careful with the instadeath blocks.

For laughs you can peek beyond the first ladder for a sight of hell. When done with it, go back and go right.
There's a life at the end of this stretch with a spike trap and bubbles just by it. Might be worth the try, might not because there's a nice chance that you'll fall down the pit.
Go down for another checkpoint, take a left until you hit the wall for the ♥♥♥♥♥♥♥♥♥♥, then go all the way to the right to get the first key. Guitar Guy can run below the deathblocks faster than the nerd. Go back to the previous floor and unlock the wall to the right.
You can instead go down the stairs for the first checkpoint then go right above the small platform, it'll fall. Fall to the right wall to skip the first key area if you want to.
Third checkpoint. The next key is just above you.
Fourth checkpoint, the moving platforms ahead should be a joke using Guitar Guy.
Five checkpoints later there's a red wall and a crystal ball switch beyond it. Use Guitar Guy to hit it and follow through to get Mike.
Once you get to the dragon there will be breakable walls to your left. They reveal a few items and two cameos.
This boss shoots waves of fire that aren't much of a problem to avoid, but they block your shots. There are also instadeath blocks flying randomly as the battles go on. Take Guitar Guy for this fight and his shots go through the fire. you can also try shooting diagonally with the Nerd for a harder fight.

Not much to say, this is pretty linear. When you hit the massive wall to your right you can switch to Mike to break it and reveal ♥♥♥♥♥♥♥♥ Man.
Two checkpoints later there's the Glitch Dwarf powerup and a life. The Glitch is pretty risky to get and might not be useful, so skip it on higher difficulties if you can't get it consistently.
Go all the way to the left and jump into the platform for ♥♥♥♥♥♥♥♥♥♥. You can jump straight right from them to get to the next checkpoint.
The boss is pretty straightforward. You can mash with the Nerd or use ♥♥♥♥♥♥♥♥ Man for more damage with less hits.

During the nearly visible blocks you can actually get a damage boost with Mike to skip. You have to let the circling ghost hit you at the apex of the jump. This might be worth ten seconds or so, but it might only be doable on easy.
Instead you can go left and take a jump at the end to find the cameo The Completionist, a Superscope, two lives and two beers.
At the checkpoint with an instadeath block above it you can use Mike to jump over it and find the cameo Pat, three lives and two beers. This jump apparently can be done by the Nerd, but it'll be way harder.
When you reach the appearing and disappearing blocks there are invisible platforms Mike can use to find ♥♥♥♥♥♥♥♥♥♥.
The boss has instadeath blocks and half the health is Bimmy's and the other Jimmy's. Take Bimmy, the hand, down first, or try to kill both together or shortly afterwards, or they'll go harder on you.

This area is linear and will use every skill you got up to now.
The only pointers to give is that during the moving platform area you jump off it earlier onto the checkpoint, and later on after the platforms that launch you up you can actually jump over the ceiling and skip that area. You'll walk forward for a while then have to jump a block to continue.
The last boss has five phases and it'll take some time to get used to the patterns, they shift at 75%, 50%, 25% and 10% of his health. The area also has pitfalls, spike traps and blocks that only appear when near you.

His first phase repeats the same route every cicle. You can stay in the middle block then move to the opposing lower corner for safe spots. Nerd can mash through, Guitar Guy can do this generally well, and ♥♥♥♥♥♥♥♥ Man can do this as fast as Nerd can mash it.

His second phase is more of the same but he gets a pair of ball shields. His cycle is still the same as the first phase but the balls will shield him against most of your shots. You can switch to Guitar Guy for an easier time as his wave shots go through the shields.

His third phase is where the speedrun starts. He will cycle through the top of the screen and will drop ♥♥♥♥ on intervals. While the timing doesn't seem to be random, the positions where he shoots are based on time alone so depending on where he was when you triggered this phase he will hit at different spots. You can stand on the higher pink blocks, jump and shoot with Guitar Guy or the Nerd. Guitar Guy shots will, again, go through the balls.

His fourth phase includes a third, pendulum ball, that'll ensure there are no safe spots in the area. Other than that, it's the same as the third, try to go through fast to minimize damage taken.

His fifth phase is more of the same. He gets a fourth ball hanging just below him so the corners where you are become even more dangerous. Try to shoot him off before he even gets near.

On easy you'll be able to practice this fight until you win. On any other difficulty you can run out of lives and be thrown back to the level selection or have a game over.

For starters, none of the cheats are benefic to your game, they're either added difficulty or a joke, and are used mainly to obtain the achievement. Using any of them once ensures the achievement. You use them at the start screen. These codes are currently set for the Xbox controller.

Konami Code: Up up down down left right left right B A Start.
This will give you a fake blue screen of death.

Hedgehog Mode: Up down left right A.
Added speed and momentum. Pretty much like the older Sonic platformers, and as such, it'll be pretty much impossible to control.

Big Head Mode: A B A Y A B B.
Big Head Mode is exactly what it says on the tin.

Easy
6 beers, infinite lives, checkpoints visible, easier enemies. Your weapon is painted pink.

Normal
3 beers, 30 lives, checkpoints visible. Your weapon is painted orange.

Old School
3 beers, 15 lives, 5 continues, checkpoints invisible, no saving. Your weapon is painted silver.
There isn't much of a change from Normal. You can farm lives by doing the hell stage up to Craig and quitting. Repeat it as many times you like. You can collect lives above the counter's cap of 15, so that isn't an issue. Remember that from now on the game doesn't save so you've got to get it down in one sitting. Getting all the lives in the way also helps to recover the ones lost through the chapters.

Hard as Balls
2 beers, 5 lives, 3 continues, checkpoints invisible, weak weapons, strong enemies, no saving. Your weapon is painted black.
The lives work the same as before. Get all the characters, reach Craig's lives and stock up before trying to complete a stage. The 2 beers actually change the game's flow and stack up the difficulty. Several areas can't be crossed with the same reckless abandon as before and the bosses become that much harder... Additionally, you cause less damage than before. About 30% less on bosses, it seems. This makes a noticeable difference on most enemies as well. You might want to run a stage without losing the Superscope for massive damage capability against the bosses. A turbo controller (or a program) becomes a must if you can't mash fast while keeping control over your directional inputs. JoyToKey does that just fine.
The hell stage boss, for example, becomes too fast to avoid and still cause damage on. It is still possible, but it might be better to make a Superscope run since there are only three checkpoints before the boss in this difficulty anyways.

♥♥♥♥♥♥♥ Impossible
No beer, 5 lives, no continue, weak weapons, strong enemies, no checkpoints, no saving. Your weapon is golden.

YOLO Is next, and I still don't know what it is. I do hope YOLO doesn't have a 1 life cap, because that wouldn't really be sane. It didn't take more than a couple of days after this guide went live for someone to finish this difficulty. Geez, good job. Additionally it was exactly what you'd expect: You need to complete the game without being hit once.

My congratulations to everybody that managed to complete this! Here's hoping you had plenty of fun!

I'd like to thank everyone that has participated or will participate in the community of this game, we could take forever without finding many of these things without everyone working together.

In special I'd like to thank:

Skatanic for the Konami code, everybody tried that one, but you posted it first as far as I can see.
evilmonkey9114 for the Big Head Mode code.
enemyplayerman for the Hedgehog mode code.
Swift Hobo for his Cameo guide, I couldn't find Keith without him. Since he's done such a good work on his guide, instead of uploading pictures to the cameos, I'll just give you his link. http://gtm.steamproxy.vip/sharedfiles/filedetails/?id=180185467#148213 Conglaturation !!, man.

All the people from AVGN for all the amusement you guys have provided and even the changes you guys caused to the youtube and internet as a whole. This game truly reminded me of how the old games were, and was fun and well made from the start to the end. Way to go, everyone, and conglaturation on your firstborn, James!