Peridot is another character I wasn't entirely sure about. From the start I was leaning towards Fiddleford but I was torn between him, Gideon and Dipper (mostly because Fiddleford would also fit really well as Bismuth and Gideon fit a lot of the requirements for Peridot). I ultimately went with Fiddleford for a couple of reasons: He's the most tech-savy side character, his glasses function as Peridot's visor, and in the show he had a semi-antagonist role (or at least serving the antagonists) early on but then developed into a loyal ally (which seemed to fit best with Peridot's arc). Not sure which name I prefer for this guy either.