Which Assasin to Raise next?

Fuuma is the best star generator aside from Jack, so if you want one of those, raise him. Cursed arm does the same thing but is cheaper and worse at it. Jekyll is not really an assassin, or at least should not be used as one because he doesn't have much to offer in that department. He's more of a berserker that takes a while to tag in. Serenity is a debuffer and offers little else, hundred faces is your (very) average arts DPS. Other than them, the rest are mediocre to bad, so I wouldn't level them unless you want to do interludes/rank-ups.

My recommendation for gameplay value is fuuma since he fills the most useful role. Personally I wouldn't level Jekyll since he takes more effort to use effectively than he's worth imo. Serenity is really only good for reducing the enemy's NP and stalling, and there are better options for both those things. There's also no reason to level hundred faces unless you really want to run an arts team with a dps assassin, otherwise anything that isn't caster will probably do better as an arts dps than she will, the choice is ultimately yours though.

I think,gameplay wise,you should focus on Cursed Arm or Fuuma because those 2 can provide you a lot of star,which is the main reason most use Assasin for.Fuuma have 2 debuff skill and a targetable dodge while Cursed Arm have on-demand crit stars skill and 3 hit dodge.I personally have Cursed Arm and I assured you that he is worth it.

For 3 star I would suggest kotaro. While for lower star, cursed arm and Samson is not bad. Kotaro can act as crit star generator and debuffer while Samson is a damage dealer + healer/cure debuff

I have max ascended my Cursed Arm and I have no complaints. He is a very solid star generator, and his entire set revolves around that. Damage wise he isn't that surprising, but that's common thanks to assassin modifier and low rarity. A minor setback, the way I see it, specially since he has a skill that boosts his critical damage and makes up for the overall damage a little. His instant death NP is actually pretty reliable, which is useful against strong non-servant enemies. His lower rarity demands very little to effectively raise him, with great results. He also has some nice survivability thanks to 'Protection Against the Wind A', which grants self evasion for 3 attacks.

People say Fuuma is a better star generator, but I honestly don't know since mine is sitting at lv 30. His set is somewhat more supportive, featuring an AoE attack debuff, a targetable evade for a single turn and an AoE debuff resistance debuff. His AoE NP could be useful for farming, so he isn't a bad choice either.

I can't say more about the others. I leveled Sanson a little since I like him, but his stats are horrific and so is his set. I heard Jing Ke is good, but Fuuma and Cursed arm are way more efficient in generating stars and overall usefulness, but I can't confirm since I never touched mine. Henry is interesting, but he shines as a Berserker and not as an Assassin. Sasaki can also be used pretty well, but I haven't used mine in a while, so I can't give much advice. I can't say anything about the others since I honestly don't know.

Either Fuuma or Hassan would do their jobs pretty well, but it's up to you. As always, it's always up to what you need or want. Level the characters you like, and don't be discouraged if they're bad or aren't viable in any way. First and foremost, enjoy the game and have fun.

Best of luck!

Fuuma and kojirou are my favorite lower rarity assassin. Fuuma has already been covered, except for the awesome animation :-)

Kojirou was my primary assassin until shiki came out. He's cheap, np does decent damage, and I like him from UBW. Also grailed to 70, and he looks better with the silver border.