A. Thief
Pros: fast attack speed and movement. High attack. Flashy skills. High critical chance and ability to backstab.
Cons: One launcher. Low health and defense. Skills cannot cancel into other skills.
B. Assassin
Pros: Three different playstyles. Physical, fire, and cold. Has debuff skills. Can hide. Tricky gameplay. Clones.
Cons: For PVP, a lot of initial skills have cooldown. Some skills require chakras in order to be used. One throw.

DOT: Damage over time
OTG: On the ground
AOE: Area of effect
X-axis: left and right
Y-axis: up and down
Launcher: skill that hits enemy upward to enable juggle combo.
KD: knockdown
SA: super armor aka tough. Allows one to keep attacking even after being hit.
PVE: player vs environment
PVP: player vs player
CD: cooldown

A. Thief
1) Assassination of Darkness
Basic special attack. Attack will bounce and hit nearby enemies. Leave at 1 for both PVE/PVP.
2) Dance of Hallucination
When kick connects, does 4 attacks while teleporting. Has short range. Will whif some hits near the sides. AOE. CD decreases in PVE as it gets leveled. Skill improves by 18% each time its leveled. Generates one blaze chakra. One of the best skills to level up when you compare damage and mp cost.
3) Flying Dagger
Throws dagger in a parabola motion. Can hit up to 3x. Launcher and OTG. Hits enemy up and away from you. CD: 7s PVE, 5s PVP. Most common followup is White Night - Gesture of Death. Rank it up to level 3 only to unlock Gesture of Death.
4) White Night - Gesture of Death
Does a forward hop attack. Does critical damage when it hits airborne enemy. Generates one cold chakra. At least get it up to level 5 to unlock White Night - Walk. CD: 12s pve, 7s pvp.
5) Blow of Focus
Dash toward enemy to break its neck and land behind the enemy. You briefly face the enemy as you land before turning the other way. The neck breaking does not work on larger enemies. You'll hit them and bounce off them. KD. Can followup with a launcher. CD decreases as it levels up. Max for more backstab opportunities.
6) Dawn of Strike
A forward dash attack that hits 12 times. Generates one cold chakra. CD: 8s PVE, 5s PVP. Leave at one point for both PVE and PVP. Rank up to level 4 if you want Hook of Judgment.
7) Tracing Chop
Disappear only to reappear with a drop attack. Aimable. KD and OTG. Generates one blaze chakra. CD decreases as it levels up in PVE only. 12s CD in PVP. Level it up to level 4 to unlock Fire Grab.
8) Hook of Judgment
Throws a hook to reel enemy closer. Has slow startup and long CD. Requires level 4 Dawn of Strike. I skipped it.
9) Air Kick
Press X and then Z to dash backwards. Useless if Evade is maxed. Rank up to 2 if you want to unlock Dark Dagger.
10) Fast Evade
Dodge to background or foreground. Max it so it has less CD.
11) Slide
Another evade skill.
12) Swear of Winds
The group attack skill. Optional.
13) Sneaky Attack
The counter attack move. Powerful but slight delay. At least one point.
14) Cancel Jump
Allows you to cancel ground moves with a jump. Another evade move. Max is good for PVP.

B. Assassin Skills
1) Physical Skill - Hide
Hide intially for 3s PVE and 4s PVP. Also grants 40% critical boost to next active skill you use and 5% increase in critical chance every time it's leveled up. CD decreases as it levels up in PVE. Remains 50s in PVP. Level up to 2 to unlock Physical Skill - Dominator of Battleground passive skill.
2) Secret Art - Black Dagger
The upgrade to blow of focus. If it connects, you teleport behind the enemy and inflict blindness for 10s PVE (6s PVP). No KD and works on everybody. CD decreases as it gets ranked up. Max for more backstab.
3) Fire Grab
Slams the enemy to the ground and can be followed up by Fire Release (press X for it). Fire release has some Y-axis range and a large X-axis range. Requires level 4 Tracing Chop. CD: 23s PVE and 15s PVP. Rank it up to level 4 to unlock Sneaky Flame. Otherwise leave it at 1.
4) Dark Dagger
Followup attack to Air Kick. A keep away move. Useless. Requires level 2 Air kick.
5) White Night - Walk
While in the air, you spray a wide projectile underneath you, which causes enemies to move and attack slower in addition to dealing damage. Requires one cold chakra and air only attack. Debuff lasts longer and is more potent the more it is ranked. It's a common followup to Gesture of Death. Max it.
CD: 13s PVE, 10s PVP.
6) Physical Skill - Leaf Slash
Slash enemy 4x. Has some invincibility frames on start up. Requires Level 1 Hide. May not connect with every slash and leaves you vulnerable if you whiff it. OTG. Not advisable to use for OTG. At least level 3 to unlock Throwing. Another good skill to max.
7) Clone Explosion - Vision Cloning Skill
You take spirit form for a few seconds and can press X to make clone explode. May have some use in PVP. Requires Hide and Fire Grab at level 2.
8) Sneaky Flame
The AOE version of Tracing Chop and does some DOT. Requires one blaze chakra. Has SA when you reappear to attack. The initial crosshair is a few feet of the direction you're facing. Aimable. CD: 15s PVE and 7s PVP. Leave it at level 1. Rank up to level 6 to unlock Phantom Sword (level 43 move).
9) Throwing
Performs two somersaults that has good X-axis (even hits behind you) and Y-axis. OTG and Launcher. CD: 28s PVE and 16s PVP. Can follow Sneaky Flame with Throwing. Max.
10) Half Moon Throwing
A really quick dash attack with great X-axis. Requires level 5 Leaf Slash. I haven't unlocked it yet. CD: 35s PVE, 20s PVP. Looks like a good max move.
11) Infinite Repel
Sends clones to attack before you followup with your own attack. Chargeable. Has a slight startup. Requires Walk level 4. CD: 37s PVE and 18s PVP. Have not unlocked yet.
12) Sudden Kill - Range Attack
As you slash, clones will attack nearby enemies. Requires Walk level 6 and one cold chakra.
CD: 40s PVE and 18s PVP. Have not unlocked yet.
13) Phantom Sword
After a slight delay, you do many slashes to enemy, which also deals DOT. Level 43 skill and requires Level 6 Sneaky Flame. CD: 1 minute PVE, 23s PVP. Not unlocked yet.
14) Sneaky Dagger
Charge attack to increase critical damage and critical chance for your clones. Requires 3 chakras and level 47. PVE only.

Only 2 skills worth ranking here.
Thief's Spirit: Increases attack and critical power. Max it.
Physical Skill - Dominator of Battleground:
Increases Critical damage. Max.

I have Explosion of Fury but it sucks. It gives 200% CD reduction in PVE only. A paltry 15% CD reduction in PVP. You also have to build a little meter, a small circle by your HP. The meter builds up only when you get hit. You're practically half dead when it the skill becomes useable.

Click to enlarge.

This is just a guide to help you decide whether Assassin class is right for you. You don't have to follow my build. Feel free to make your own custom build. I hope this will be a good reference. I'm more of a PVE person and my Assassin's name is FlatChest.