Tama Lancer team

If you want damage, go for Scathach Assassin.
If you want more utility and support, go for Kotarou. A targetable Evade is not common.

You can also try Sanzang (from a support if you don't have her) in the mix : she will increase NP gain and Star gen of the party and she can protect Tamamo from her stun debuff (Sanzang's third skill needs to be activated before Tamamo's). As a caster, she has a low Star weight so all the stars will go to Tamamo.

Kotaru is a more consistent star generator while Shishou can generate a mass amount of stars in short bursts (reliant on her NP). Although Shisou will output far more damage than Kotaru in the long run, she's not the main damage dealer. With stars being consistently generated by Kotaru (and Tamamo to some extent [she's great at generating stars too]), the damage differential between Kotaru and Shishou is minimal thanks to Kotaru having a better chance of critting throughout any given node. That being said, in the event that Tamamo has only one card available in a hand, Kotaru can still manage to deal a modest amount of crit damage while amassing a surfeit of stars for Tamamo during the following command phase.

I don’t think damage should be a major consideration. First, Assassins deal low damage in general. Secondly, this is a crit generator Assassin for a Tamamo DPS team, not a let’s-kill-Ozy-with-Assassins team. Your Assassin will probably only be using Quick cards (lowest damage out of all card types) and maybe Arts, if there’s nothing better. In short, whether you clear a node depends on how often and how hard Tamamo crits, not whether Assassin A deals marginally more damage than Assassin B. So, the key Assassin criteria are star generation, support utility, and survivability.

Fuuma is probably the second-best star generator in the game after Jack. It’s difficult to imagine any scenario in which Scathach comes close to matching him, even without considering Tamamo’s insane buff on his star gen. He has Presence Concealment A+ vs. E, four-hit vs. three-hit Quick cards, and they both have five-hit AoE Quick NPs. His targetable Evade is an excellent buff, and his enemy crit/damage down and debuff resist down skills are also quite useful to the team. He is a bit squishy, with around 2,500 less HP than a level 80 Scathach, but I don’t think that’s enough to tip the scales in her favor.

My other question for you: what is your strategy for ensuring Tama gets the stars? She has below-average star weight, I’d say, and no skills to boost it. Most good crit servants either have natural star weight due to class (Ozy) or get it from skills (Raikou, Jalter, Gilgamesh, Lancelot). It’s not an issue if Fuuma is consistently generating 40-50 stars per turn, but I’m not sure how likely that is. Do you have a star weight CE to give her?