User blog comment:B214/Volume 22 spoilers/@comment-104.230.254.245-20160819121246/@comment-9595671-20160819195442

Makes sense, she's probably second-in command after him and as the Strongest Woman in Heaven is pretty much the second strongest Angel.

Michael himself probably had to aid in sealing Trihexa rather than staying behind because he may have known something important that required him to actually be part of it.