Marcus being jealous of someone having too much of Percy's attention (Oliver, Cedric. Penny anyone you want really)
or as an alternative (or in combination up to you)
Marcus wanting to terrorize the twins when he learns about the pyramid incident.
Name
EliseDonut
Likes
Marcus being protective, Marcus being obsessed with Percy, Percy being touched by how much Marcus cares about him.
Dislikes
rape / non-con, scat, vomit, self harm, Marcus being Percy's bully first (he can be a bully to other people though if you want.)