Hm...

Hayate Gekko - Lightning

Mystery: Dance of the Crescent Moon
Create three Shadow Clones. Next turn, if at least 2 clones are alive, this skill can be activated again to deal damage and Knockdown to one opponent and remove shields.
Cooldown: 3
Battlefield Cooldown: 1
Cost: 20

Standard: Kenjutsu Attack
Attack front enemy, chance of triple combo and repulse. (Like lightning main standard)

Chase: Crescent Moon Slash
Chase Low Float, cause Knockdown.

Passive: Meld Into Shadows
If Hayate takes lethal damage an a clone is alive, it will die, he will gain 10% of his max health, and his Mystery cool down will go down 1.

Passive: Ill Boy Jonin
Hayate begins the game with Poisoning. If Hayate has a debuff, his Attack and Ninjutsu will raise by a certain percent. This skill can stack twice.

I tried. I dunno if this is OP or absolute *, but it was pretty fun to make.

One of the things I always wondered is why Kakuzu was so squishy. I also wondered why he wasn't wearing his cloak.

This should solve it:

Kakuzu (Initial encounter, Asuma's death)

Mystery: Heart Steal (40 chakra, 3 cooldown, 2 battlefield cooldown)
Cause heavy damage and Knockdown to target enemy. If this skill kills an enemy character (not a summon or puppet), they cannot resurrect, and Kakuzu gains one "Heart".

Standard: Tentacle Fist
Same as regular.

Chase: Immobilizing Grasp
Chase Repulse, cause Low Float and Immobile. Can be activated twice per turn.

Passive: Stone Skin
Before each action, Kakuzu gives himself a shield based off his Resistance. Can be activated twice per turn.

Passive: Five Hearts
Kakuzu's total life becomes 1/3 of his Life stat. He starts the round with one spare Heart. Whenever Kakuzu dies, he loses a Heart and revives. Kakuzu can hold up to four spare Hearts.

She's cool, but I personally think she has too much "uzumaki" and too little "Tayuya". What happened to her flute and stuff?

Gozu, the Demon Brother (one of the guys who attacked Team 7 in the Land of Waves)

Mystery: Chain Wrap
Causes immobile and destroys shield of one enemy. Can only be performed if Meizu is alive.
40 chakra
2 round cool down

Standard: Poison claw
Chance of 3 Combo, Repulse, and Poison.

Chase: Chain Shred
Chases the immobile unit, destroy shield and cause Low Float. This chase has high damage.

Passive: Water Melding
Chance to dodge increases. Whenever a dodge is performed, give yourself a small shield.

Passive: Double Team
At the beginning of battle, summon Meizu. Meizu has all of Gozu's skills.

Interesting twist on the character.

Gato
Mystery: Hire Henchmen (60 chakra, 2 round CD, 2 battlefield CD)
Sacrifice 10% of your current life to summon two Demon Brothers. The Demon Brothers will activate their Mystery automatically. When used with Demon Brothers alive, will summon a copy of Zabuza .

Standard: Making Money
Heal yourself a little and gain 10 chakra.

Passive: Bodyguard with Eye Patch
At the beginning of the fight, summon Waraji. Waraji will activate his Mystery, "Sword Drawing Jutsu", automatically at no cost every 3 turns to deal damage and remove shields to 1 opponent. He also has a standard attack that can cause Repulse, and can chase Knockdown and cause Low Float.

Passive: Bodyguard with Hat
At the beginning of battle. summon Zori. Zori will activate his Mystery, "Speed Slice Jutsu", to give himself 1 extra attack automatically at no cost every 3 turns. He also has a Standard attack that has a chance to cause Repulse, and can chase Repulse to Knockdown.

Passive: Screw the Rules, I have Money!
If your total amount of Coins is greater than your opponent's, all of Gato's henchmen will have their Mystery cooldowns lowered by 1. Also, Gato will receive a shield.

(I tried.)

Here's another one:

Might Guy
Mystery: Front Lotus (20 chakra, 2-round CD, no Battlefield CD. No Prompt.)
Deals weak damage to one enemy, causes Knockdown and Immobile and Ignite. Can only be used with one Gate open.

Standard: Nunchaku Taijutsu
Similar to normal Guy's standard. Chance of 4 Combo and High Float.

Chase: Dynamic Entry!
Chase Low Float, cause Repulse and Interruption. Can be activated twice per turn.

Passive: Single Gate Release
Before each action, Guy will lose 10% of his total HP, but will gain a moderate Attack, Defense, and Resistance boost. This can be triggered up to seven times total. After 2 Gates, Guy's mystery will become "Fiery Kick"; after 6 Gates, Guy's mystery will turn into "Morning Pea*", and after 7 Gates, it will become "Daytime Tiger".

Passive: Youth! Never Ends!
At the beginning of combat, increases Taijutsu attack of all ninjas on the team. If another Jonin is present, increase this buff by 1.5%.

Other Mysteries:

All of these are non-prompt.

Fiery Kick (40 Chakra, 2-round CD, no Battlefield CD)Deals damage to up to 2 enemies, causing Low Float and Ignition to both.

Morning Pea* (60 Chakra, 3-round CD, no Battlefield CD)
Deals heavy damage to one enemy and causes Interruption and Ignite. Every other opponent will also get Ignite.

Daytime Tiger (100 Chakra, 5-round CD, no Battlefield CD)
Instantly kill one opponent and deal moderate damage and Ignite to all other opponents. This attack cannot be dodged, and opponents killed by this Mystery cannot revive. After performing this Mystery, Guy obtains Immobile for 3 turns.

I didn't realize someone else played Fairy Fencer F... might make my own version of Fang or someone later. Cool.