[@Ebil Bunny] I mean like a character that contrasts with another character, a literary foil. Sure, they have their similarities, but they do also have their differences. Take for example their magic.

Both have magic that involves a person and their beliefs to some extent, however both are mechanically different in how they work. The psyche magic he utilizes seems to focus on altering the minds of other creatures and people, whilst his own magic is mainly centered around altering the environment and himself.

Another is how they both view their "main" type of magic - Theo ultimately shuns his psyche magic but Klaus doesn't have a problem with his, though he does feel it is a little lackluster in his mind.

Another is the imposter syndrome. I would sort of say that Klaus might have it, but that's besides the point. Klaus won't go out of his way to verbally hide it and instead would probably verbalize it openly, possibly getting angry if someone says otherwise. Theo does the opposite from what I've seen and goes out of his way to dodge and hide the question of himself being possibly inept behind a smile and such.

Those are some of the few that I can think of off the top of my head. They have their similarities, but those similarities might be what accentuates the difference between em.