Iris

Little is known about her as of yet, other than that she was like an older sister to Hilda. Iris gave Hilda a necklace and asked her to take care of Beel if she was to ever pass away. The Great Demon King once painted a portrait of her, but out of embarrassment, he created an artificial demon to stop anyone from seeing it (despite it actually being quite good). Hilda mentioned that Beel probably doesn't remember her but according to the manga on chapter 168 page 19, Hilda mentioned that she would be pleased to have Beel meeting his mother one day. But when Oga asked Hilda whether lris is still alive, she remained in silence. The Great Demon Lord instructed Hilda to become Beel's "mother", implying no one else at the time was able to fulfill that role.