How should I use my enemy skill materia?

[Jiro]

Uh...catch 'em all...?

Got it in one

But yeah, this is what happens: When an enemy uses a skill that can be learned via the the Enemy Skill materia, it is learned by the character who has it equipped (and is hit by the move). When you transfer that materia to another character, they will still have that ability in the materia, even if they haven't been hit by that move previously. It's all about the materia, not the character!