Magnus Bane, the High Warlock of Brooklyn, proves that Downworlders aren't that bad. If you needed any other reasons to convince you that Valentine's Downworlder extermination plan is a horrible idea, Magnus Bane can give you a few.
If there were a Real Warlocks of New York show, Magnus would definitely be the witty one who imparts sage words of wisdom to his friends. (In between all the throwing curses and telepathically flipping tables and tossing drinks in each other's faces.)
We learn a few things about Downworlders from Magnus Bane. He's a warlock, which according to Hodge means he's one of "Lilith's children, being half-demon" (5.236). That sounds like a bad thing, and to be fair, the dude has no navel, which means he probably wasn't born in any sort of traditional manner.
Then there's the whole mind-block ordeal, putting a block on Clary's memories so that she can't see or remember anything Shadowhunter related. We find out that technically it was her mother's idea. He was just doing what he was told. And he's quite fond of Clary, telling her, "You're the only child I have ever watched grow up that way, you know" (13.11). Aww.
What else do we know about Magnus? He also has an appreciation for travel (well, he was in Tanzania for some reason) and the arts, quoting Oscar Wilde. It's incredible that he grew up to be so sensitive given his difficult childhood, with a mother who committed suicide and a father who tried to drown him at age ten. Ouch, talk about a rough childhood. Poor Magnus has hated himself ever since.
At least Alec's sweet on him, telling him, "You can't help how you're born" (13.28)—whether you're gay, a magic-casting demon, or both! Magnus later returns the favor, nursing Alec back to health after he's critically wounded in the fight with Abbadon. It's adorable, even though Magnus is heaven knows how many years older than Alec is, having been erasing Clary's memory since her birth fifteen years ago.