From the now deleted Friday Night Funkin' wiki:

Darnell's voice can be found here.

Darnell is a trigger-happy person who is characterized by his pyromaniac tendencies; he loves setting things on fire so much to the point where he even goes as far as to burn down 9 buildings to watch them burn for his own pleasure, giving him a near sadistic nature. Darnell is very intelligent and has a high intellect, as he is able to understand many things with little effort. Like Nene, Darnell appears to be vindictive and petty, attacking his own friend Pico for allowing his relationship with Boyfriend to ruin their contract with Daddy Dearest.

That being said, Darnell isn't devoid of redeeming qualities. When encountering Pico after the latter tries escaping from Nene, Darnell offers to rap with him over his life rather than trying to kill him on the spot like the former did, showcasing an honorable side to him. Furthermore when Pico wins their fight, rather than holding any ill will towards his former classmate, Darnell simply smiles and embraces him, showing how much he cares for Pico as a friend.