I recently got Fionn on my main, should i raise him now ? or should i continue to max Taiga and Proto Chu instead ?

go for Taiga, Fionn aint that good rn

Gameplay wise, the Cus are better. Taiga too, though her NP would lack power since she's harder to get to NP5 since she doesn't appear in the FP summon.

Fionn does get better but until that time comes stick with the Cus, and Shuwen.

As for skills, level those you use most, level others to 6 to at least take 1 turn off cooldown.

Sadly there really isn't anything Fionn can do that another lancer of equal or lesser rarity can't do better, at least not until he gets his strengthening quests. He becomes a semi-decent support, tank and AoE lancer after that. Keeping that in mind, I personally chose to raise my Fionn.

Sadly, u shouldn't. Santa Jalter can do anything he can do but better now since she is NP5. He only start to have a true usage in JP server after his buff on skill 2 on Jan 2019 which makes him an Art based 3T lancer surfer, but not anything else apart from that. Unless u have or plannning to pull Tamamoo and is willing to wait 2 years for his buff, there is no reason to use him.

If you don't have any major attachment to him or want to "catch them all," then you'd be ok with just burning Fionn at this point with the lineup of lancers you do have. Everything that he can do is done better by your existing lancers, and it isn't until very far in the future where Fionn gets a rank up, and it really doesn't do enough to make him compete with the ones you listed. (imo, do not take this as absolute truth)

Fionn was created to be rare prism fodder. Burn him and buy Mona Lisa!