OMG!!!!!! Brother Zachariah!!!!!
A parabatai. Like he was. And Jace knew, too, what that faded rune meant: a parabatai whose other half was dead. He felt his sympathy leap toward Brother Zachariah, as he imagined himself without Alec, with only that faded rune to remind him where once he had been bonded to someone who knew all the best and worst parts of his soul.