Sanctuary
Member
Esther can use any familiar. Anybody can use any familiar, just that some types get certain bonuses with certain characters, but I don't think they're really that important.
Yes, all characters can use all genus types. I should have specified that it's one of the few that are her genus favorites. The +10% stats might not really matter in the end since you can simply level up a few more times if you really need to I suppose.
Still, I'd rather keep them attached to their preferred type, just because. She also has a large mana pool, so it's good to have a caster type that has come kind of healing as well as offensive power (which her default familiar does). For the times that's not needed though, she needs some kind of really good physical damage dealer. Until MUCH later in the game, only Griffy fits that, plus he also happens to have some sick area of effect spell damage on top of it.