Zero is at his core a ranged, mixed Damage Dealing, Crowd Controlling, Healer character. His kit revolves around controlling the waves, securing yourself and your team at the same time, and generally putting in his share of damage.

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Damage
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+Slow but consistency, steady damage on singular or multiple targets in a line; able to spread on hit effects rapidly from weapons
+A baked in high speed, fast animation, lower cooldown burst shot that can be improved
+An Ultimate skill option capable of further pumping out high amounts of damage with some finesse
+Alternatively capable through skill selection of wide area of effect damage dealing, very quickly, at a decently high base values

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Crowd Control
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+Your 2 bread and butter skills are both fantastic at shutting down minor and medium foes with minimal threat to you
+Skills cover an area of effect on use, with the ability to skill into even stronger options for such

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Healing
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+Dealing damage with primary attack builds your gauge to deploy medpacks on the field for you or allies to pick up
+Skills can be taken to further increase potency, area of effect, or granting temporary immunity
+An Ultimate option that can be used as an emergency, smaller value heal but with a number of buff options

===The Medic ===
If you like supporting and healing, or you just have a capable team of blowing through things while not being the best at avoiding damage, here's the easy go-to for each level; with alternative options and reasoning based around team capabilities with emphasis on Zero's ability to be flexible.

LVL 2
++Enhanced Healing Kit; By far the best option and will keep itself relevant by the end of the stage. After a short delay (visible under the medpack as a UI element) the pack will 'upgrade' itself into a better version, healing for more HP. While it requires that second or so making it difficult in a pinch situation, we have gadgets to fall back on for those as a 'worst case scenario'

+Recurring Trap or Wingmen; Neither of these options are bad, in fact it's a curse in disguise that he has 3 generally good options all bunched at the same level. If your team is good at damage mitigation or avoidance then either is an acceptable pick. I personally prefer Wingmen for the added damage and area when faced with the choice

LVL 4
++Brief Respite; 1 Second of immunity after picking up a healing kit, simply and straight to the point. If you have kits placed during a boss or elite fight you can completely negate some effects with timing just by picking one up

+Trap Implosion; After dodging through your trap field it sends out damage shockwaves both forward and behind. An excellent option for area damage and in general probably the best skill on this level but with the caveat that it requires your dodge to activate it. At lower levels this isn't really a problem but in the higher ones missing a single dodge can be lethal and if you happen to dodge into something or at the wrong time the damage you send out won't be worth your life

-Force Lightning; Cool name, and if your an older version player a movement action you'll have hard coded into you, but a terrible skill for healing focus. After dodging your next attack within 3.5 seconds will instead be a cone. It actually deals great damage, but it requires your dodge before use, holding the charge deceptively short lived, and is fairly short ranged. The biggest sin of this skill however is it does not generate charge towards your Healing Kits; I do not know if this is a bug or intended but is a no-no at any rate for this build because of it

LVL 6
++Self Medication; I actually dislike this skill for the randomness of the drop pattern of these pills upon skill activation, but the ability to have some on demand minor heals with random buffs applied it fits with the motif at any rate. They can be hard to spot, they can spawn outside your current play field, in walls, into the void for all I know so this is a skill with some difficulty to recommend at times but is still useful

+Event Horizon; Another not bad option, but another problematic one for it's own reasons as well. As the damage dealing option of Ultimates you can grab this if your team is managing well, or you just like it more. Note that this skill has a long start up animation, one that locks you in place and doesn't prevent enemies from squirming about you or moving. The ball it releases is slow, with only an 'OK' size, making it pretty exclusive to bosses, elites, or tight corridors

LVL 8
++Healing Area; Turn your kits into an area effect heal around them, able to support an entire party around a kit rather than one at a time and it shares synergies with the other skills as well

+Damage Area; Another fantastic option if your party is handling itself well but still needing a little bit here and there. A close range option, but with a strong base value of damage and knock back. Not for every situation though as you'll want to use Healing Kits offensively with this, and might end up 'wasting' the actual kit if the party doesn't need it at the time.

-Re-Energized; Not a bad skill, but a lesser choice for healing focus. If your party doesn't need healing as much, and you're not a fan of Damage Area, this might just be for you. Requires some finesse and proper positioning however in shooting through your trap field but you will get used to it

LVL 10
++Wave Trap; Triggering the Laser Trap sends out shockwaves both forward and back dealing damage. To round things out adding some area damage into the build isn't bad at all, and this skill gives you an emergency fall back with some fangs to back it up

-/+ Addiction/Steerable Superball; Here's where things get messy; your Ultimate skill upgrades. Addiction is in a word, crap. All it does is add 5 additional pickups to the Ultimate, nothing else. The ability itself isn't strong enough to warrant such a simple and barebones addition. Steerable Superball by comparison is fairly decent but not really my favored choice, however I recommend experimenting for yourself to find what works best for you

?Photon Burst; IF you took Re-Energized then the capabilities of this can soar, but ultimately this skill will create additional animation time onto your ability which can lock you into place. You'll need to be much more aware of your positioning and timing with this

===The Photon Sniper ===
Long range damage, strong blow back potential on small or medium foes, high burst. Healing Kits are there to help your team but you aren't there to keep them all alive if they can't keep out of big spicy red areas on the ground. Very little wiggle room needed on skill choices here.

LVL 2
++Wingmen; Make your focus skill even stronger and bigger, what's not to love? No other options needed to be considered here

LVL 4
++Force Lightning; For the foe that gets past everything else, blast em in the face to keep them at range

+Trap Implosion; Still a strong option regardless of the circumstance but with the way we'll be positioning around our traps later this can make things messier than they need to be

LVL 6
+Event Horizon; Another linear attack you can throw, with elite or boss melting capabilities. Just be mindful of your spacing and timing with it

+Self Medication; Earning a honorary positioning here just for the fact that pill buffs can boost your damage

LVL 8
++Re-Energized; Drop that trap field and blow your foes away, firing your Photon Bolt through the trap powers it up for even more damage. This will NOT empower Wingmen however, only the center bolt you fire. This WILL however empower our final skill, Photon Burst

LVL 10
++Photon Burst; Fire 2 additional bursts after your initial cast of Photon Bolt. Neither of these extra casts gain additional Wingmen bolts, but they will be empowered by Re-Energized as stated above. Turn any corridor or line into a target barrage space, and from a considerable distance away

===The Trapper===
Crowd Control and damage all wrapped up into one nice, neat package; you. Positioning is important for this loadout, and you'll need to be prepared to scrap in close for best results.

LVL 2
++Recurring Trap; After being consumed your Laser Trap will re-arm itself after 4 seconds. Laser Trap lasts for 13 seconds on the ground, giving you on average 1-2 activations off this skill. When used aggressively this skill can get you 3 re-arms per trap use, if it's used on activation each time

+Wingmen; A suitable alternative for area CC if you're looking for variety or playing to item buffs

LVL4
++Trap Implosion; Here we freaking go, prepare your gpus because this skill right now is your dedicated lag the game button, and it'll only get worse later down the road. Dodge through your trap line and send out damaging shockwaves. High damage, fast, keeps you positioned around your trap line, no need to look at the other options here

LVL 6
++Event Horizon; You'll be fairly close to the action anyway, so having a high damage frontal attack is right up your alley

+Self Medication; Because you'll be closer to the action you might start taking more damage as well, so having an option for quick heals isn't the worst but generally gadgets will fill in for that as well

LVL 8
++Damage Area; Dancing around with your traps means you won't be building as much energy but having this on hand to blast close foes is never a bad choice for some breathing room. With all the Trap Implosion proc'ing you'll be doing you won't get as much use out of the other damage option on this tier but it doesn't make Re-Energized a 'bad' option either

LVL 10
++Wave Trap; Another no brainer here, more shockwaves, is more damage, is more frame lag (please for the love of all things hope this gets patched sooner than later lol). Each reactivation from Recurring Trap refreshes this proc ability as well

===Weapons & Gear===

As far as equipment goes you take what works best with you, but there are some standout weapons and bonuses that will benefit you depending on how you like to play most.

Thundercall; Makes your energy pool larger, a larger pool means additional Healing Kits that can be stored up at a time




Stinger; Passive Crit chance, and critical hits makes your next skill in a short time hit even harder. Power up those snipes




Barracuda & Ohgur's Lament; Both are options that control the battle, stunning or slowing enemies to let you better take care of them on your terms rather than theirs




Notable Weapon & Gear Effects
++Extra Healing Kit Effect, Laser Trap Speed Buff, Upgraded (Laser Trap / Photon Bolt), Cooldown Reduction (Laser Trap / Photon Bolt), Noxious, Lifesucker, Right Here Right Now, Unerring


===Talisman===
As a still yet new effect there's going to be all manner of combinations and effects, so this section will update as best effects are found so this is just what I've discovered so far as working well

Idol of Future Conquerors; Currently bugged for Zero as no matter what I do the only thing that seems to proc it is Event Horizon. If it gets fixed however the ability to rapidly re-trigger photon bolt or trap will be huge

Fortune Sticks; 40% at full upgrade to not consume a gadget on use, pretty useful for supporting or damage dealing

Symbol of Swiftness; Faster dodge regeneration, better survivability and in the case of Trap Implosion, more procs

As with anything, what you read here is purely personal preference and tips; I'm hardly the authority on this but I noticed the Steam needed some up to date character guides with the blossoming reviews bringing in potential new players. I've been playing a bit now too and the latest changes made things a little more streamlined in some ways, and a bit more spaghetti in others so as the game continues to evolve and release new options might open up or choices might change.

At any rate if you got this far thanks for reading and I really hope you enjoy your time playing!