Hits required for a blue samurai: usually three, sometimes four
Hits required for a red samurai: six

Suzume is the worst ninja in the game. She is on the weaker side when it comes to melee damage, and her special attack is pretty much useless. All it does is that it stuns enemies, which you can do in many other ways without wasting unstable energy.
I recommend not playing her at all, since there is a better option for every situation.

Hits required for a blue samurai: two
Hits required for a red samurai: usually four, sometimes five

Kunoichi is on the stronger side when it comes to melee damage, and her special attack is pretty good, and it is useful against groups of enemies.
She is a good character, but just like with Suzume, there is always a better option.
You can absolutely play as her if you want to. She is not bad, but she is not better than the remaining ninjas.

Hits required for a blue samurai: usually two, sometimes one
Hits required for a red samurai: usually four, sometimes three

Futo is a good character, and just like Kunoichi, he is on the stronger side when it comes to melee damage. However, he is more useful when it comes to dealing with spear enemies. He is also necessary for dealing with the big Samurais, but his special attack is not very useful.
You will have to play as Futo, but you should play as other characters when you don't need him.

Hits required for a blue samurai: usually two, sometimes one
Hits required for a red samurai: usually four, sometimes five

Tora is very good for melee combat. Not only is he on the strong side when it comes to melee damage, but he also attacks very fast. His special attack is only used for moving quicker though, and that's not very useful. He also has another drawback, and that is that he can't jump or climb as high as the other ninjas.
Tora is very good, and you should absolutely play as him.

Hits required for a blue samurai: four (one)
Hits required for a red samurai: eight (three)

Shun is actually the weakest ninja when it comes to damage, but that is because he uses a bow and arrow as weapon. He is the only ninja with ranged attacks, and that is his strength. He is very useful when trying to sneak through a camp as he can shoot enemies down without having to get close. Spam-clicking the attack button is the weak attack, but holding it down and aiming deals more damage.
His special attack is also very useful for attacking groups of enemies.
I recommend playing Shun a lot.

Hits required for a blue samurai: usually three, sometimes four
Hits required for a red samurai: six

Hiro is actually on the weaker side when it comes to melee damage, but that is not a problem since he is the only ninja with the ability to use Kuji Magic. He can use spells to deal massive area damage, to hide, to defeat groups of enemies with one hit and much more. His special attack is also very powerful, both for defeating groups of enemies, and for getting to places that can be hard to reach otherwise.
His attacks are a little weaker, but the spells make up for that. The other ninjas don't even get close to Hiro's power.

Most players will play Hiro the most since he is the most powerful and versatile ninja. He is always recommended, and he is the only playable ninja during boss fights.
If you want to focus on melee combat, play Tora, since he deals high damage. Futo also works, but he's not quite as useful. Same thing goes for Kunoichi.
If you want to focus on ranged combat, play Shun. He is the only ranged ninja.
You can play Suzume if you want to, but she is the weakest ninja, and she is not useful in any situation.

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