Some might say that given this character's relatively dark aesthetic even before being possessed by a demon, that there wasn't much that needed to change. But the subtle changes are there. The stubble, the messier hair, the more formal clothing, it's all very much out of character for Jace. Jace is already a pretty dark character to be sure but he is still basically just a boy, barely into his 20s. Before the possession, all of his features and style points to his youth (read: innocence) the demon corruption pushes him farther into adulthood hence the facial hair, longer locks and crisp suits. Still hot though.
Seelie Queen: You look stunning as always. Edom’s done wonders for your complexion. Lilith: And you are clearly eager to send me back there. Seelie Queen: Don’t be silly. Endowing Eve with Seelie beauty was all the revenge my people ever needed.
Clary: If you were at Lake Lin, and it was Aline instead of Jace, what would you have done? Consul Penhallow: I know your mother raised you as a Mundane, with the idealistic belief that love comes above all. If I’d been at Lake Lin, we’d still have Raziel’s wish. Because being a shadowhunter is about sacrifice. That’s the one thing you’ve never understood, but maybe now you will. [She steps back] Consul Penhallow: Clarissa Fairchild, you are hereby sentenced to death.