Issac

Issac was the first target Zen and Natani had in their Assasin - work, Issac is reason why Natani thinks she's a male because Issac used a Forbidden spell that destroyed part of Natani's soul, which Zen repaired by combinating his soul with Natani's (Natani is "he" because of this) the spell werent Forbidden for nothing, after Natani was "alive" again, they heard that Issac died at casting that spell, and their mission was agreed a completed, Zen and Natani feeded the rumours about them being so good or afraid assasins that Issac killed himself, when he saw them at first, this is sort of why Zen and Natani are called "Magi Brothers" too.