The real Crew is the son of a sorceress in a world where magic is pretty commonplace. He had a lot of arcane potential in him, but he got a disease called endrena that deteriorates your aura and makes magic almost impossible.

Crew has to make do with very limited abilities, but makes up for it by being knowledgeable about magic’s limits and loopholes. He beats a lot of stronger opponents by exploiting weaknesses in their spellcraft and outsmarting them. Think Joseph Joestar but with zero charisma.

Carbon Crew is from an alternate timeline where Crew never contracted endrena and lived up to his potential in the worst way possible. He’s made contract after contract with various malevolent forces and now he’s a walking vessel for dozens of demons.

Carbon has no regard for others. He’s gone without facing consequences for his actions for so long that he can’t see people as anything but toys. He gambled with his soul and won so many times that he thinks he can never lose. His body is bursting with demonic energy.

Carbon and normal Crew don’t get along, but Carbon has an antagonistic attraction to Crew that results in a lot of fucked up scenarios. They’re afraid to kill each other because they don’t know what will happen if one of them dies, but they still come into conflict frequently.

I just imagine that with all the reality-warping Carbon does he occasionally ends up in Crew’s timeline by chance and starts fucking things up, and Crew has to get rid of him without actually killing him. So that’s why they’re together sometimes. The end.

-Source