Magnus Bane (Harry Shum Jr. on the Freeform series) is the High Warlock of Brooklyn. Being over four centuries old (although he often lies about his age), his life has intertwined with several Shadowhunters and Downworlders over the centuries.
He regularly tampered with the mind of Clary Fray after designing a spell that would erase her memories of the Shadow World once every two years as a favor to her mother, Jocelyn. He later has Clary retrieve the Book of the White for him in City of Glass and agrees to help revive her mother with it.
The books describe him as a "freewheeling bisexual."
Despite his flamboyant appearance and whimsical personality, Magnus is a kind yet jaded person who yearns for love and acceptance but has great trouble revealing his honest desires and personal secrets to others due to his long and traumatic past. His life has frequently been entangled with the fates of certain Shadowhunters (specifically the Herondale family during the Victorian and Edwardian eras and the Fairchild family following the collapse of Valentine's Circle), but he doesn't become seriously involved with the workings of their society and their struggle for survival and reformation until he meets Alec Lightwood, who goes on to become the love of his life.
He is a flamboyant dresser who dyes his hair and likes to wear bright makeup. Magnus Bane is very open about the fact that he has loved many people and creatures regardless of gender or species.
He tells Alec, "I've been alive for hundreds of years. I've been with men, been with women — with faeries and warlocks and vampires, and even a djinn or two."
Alec and Magnus kept their relationship a secret because same-sex relationships were not allowed within the Shadowhunters. Their secret was revealed when both characters kissed in front of Shadowhunters, Downworlders, parents, and friends, cementing their relationship.
The two are married in the season finale.