Scathach Assassin

Shishou does more damage as an SR but most of her star gen is tied to her AOE np. She also has thicc thighs which fuuma doesn't have ;)

Fuuma on the other hand has piss poor damage but a good support kit including enemy damage down as well as a targetable stealth. He is however the second best assassin star gen in the game right now due to a high number of hits in every card in his deck.

I'd level both. Shishou first because ascension materials are readily available but fuuma as soon as you can as he is a very good star gen

Hmm I'm still fairly new to this game but from my understanding, Scathach Assassin is great for gaining and doling out crit damage so she would be a great servant, however, if used properly Fuuma can debuff most bosses like crazy and open the path for your more powerful servants to wreak havoc.

There are probably only a few assassins in NA right now that are worth investing into. Fuuma has his rights as a star generator, but won't perform as well as say a 4* or even 5*. For example, even though Scathach Assassin has an all around kit that doesn't really benefit her, she still lives longer and hits harder than Fuuma. I'd say to wait for rate ups on the assassins near the top tiers like Jack if you really want a good assassin. And about those embers (which I'm thinking is EXP), you can and should use them on anyone regardless of their class matches the ember. It's better to improve your servant's level there and now than squeeze out some EXP from class bonuses. Good luck!