Mutsumi Ashitaba is also known as 明日葉六実, Muukun.

明日葉六実
Born Souma Mutsumi, in 3 March 1991, he lived most of his life at Aizu with his grandmother, after the car accident that took away the life of both of his parents. He is very taciturn, but not shy nor introvert.
After marrying Hinata Ashitaba he acquired her surname.
He is known in her family as Muu-kun, or Muko ("Groom")