Magnus Bane, The High Warlock of Brooklyn also seems to be the only warlock in Brooklyn. Whenever the gang needs magic, he's the only one they call. He's dating Alec, although we have no idea why, since Alec can't get over Jace and often denies that he likes Magnus, saying things like "We're not dating" (2.13.143) or "He's not my warlock" (1.7.81), which sounds like a bad '80s sitcom: Not My Warlock! Starring Alan Thicke and Markie Post. Cue canned laugh track.
We would feel bad for Magnus's feelings, but he's 300 and dating a 16-year-old, so he should expect this.
Magnus heals Jace, Maia, Luke, Isabelle, and pretty much every other character in the book at some point. After the Silent City is destroyed by Valentine, he promises the Inquisitor that he'll hold Jace prisoner in his apartment. He's the worst jailkeeper ever, pretty much giving Jace free rein of wherever he wants to go. Magnus's strengths include magic and fashion, being a good warden is not among them.