Magnus is Saga's father. He's a human and he comes from Scotland[1].
After Etta's disappearance, he seems to be increasingly distraught, abandoning most of the things he has enjoyed in the past (such as playing a guitar, crafting, or taking photos), focusing only on keeping Saga safe, which leads to somewhat strained relationship between the two, given Saga's rebellious nature.
It is suggested that when he could not keep his daughter locked up any more and Saga left the House of All Worlds to travel, he himself eventually did leave his safe place to search for his wife; yet while Saga got to say her goodbyes to him, she never knew whether Magnus eventually found Etta again, but as an older woman she liked to think they did eventually reunite, someplace green and beautiful.