Okay, I've come up with two possible quirks for my character - 

1.) Iron manipulation. He can control and manipulate iron atoms, so that means he can control iron alloys, and that in turn means blood. However, it requires the use of his own iron atoms within his blood, so if he tries anything too stressing, the iron atoms in his blood explode, causing damage to his heart and liver, and severe illness. 
2.) The ability to create and control a large dog-like being, made of hardened shadow. It can be used to defend him or attack those that displeases him. Due to it being made of pure shadowy substances, if he creates and controls it for too long, his vision begins to severely worsen, and if he doesn't stop, eventually permanent blindness.