I'm picking 'Stolfo for sure, unless I somehow pick him up before then. I was considering between him, Fran, and Gorgon, but I like how his set works from playing with him as a support. It's... bad, but fun bad.
If you're looking for a support caster, maybe consider investing in an Amadeus. He's extremely cheap to build and I've really been enjoying him. He's basically a two-trick pony since his only redeeming features are two of his skills, but they're incredibly powerful skills that would break the game if they were on an otherwise competent character. Protection of the Music God is a powerful party-wide Arts buff that's just about the same level as Tamamo's (although only for one turn), and Eine Kleine Nachtmusik grants fifty critical stars and guarantees party-wide crits for a turn. I've been playing him with Combat Uniform and Golden Captures the Carp and he pulls a ton of weight. Just don't expect him to do like, literally anything else. His NP is tolerable if he ever gets to it, but mostly he just wastes deck space and dies aside from his niche. But it is a great and fun niche. Especially if you're already using or pairing with a Merlin since he has great synergy with Hero Creation.