She is Akira's childhood friend who has feelings for him and likes his new tougher persona. She is jealous of Ryo Asuka thinking Ryo's a chick when Ryo wants Akira to go with him.

She gets unholy things done to her by a demon at the beach but Akira uses his powers to make her think it was a dream. Near the end of the manga, Miki and her family are killed by crazed townsfolk who think Miki and her family are demons and her head is put on a pole. When Akira finds this he murders everyone there and holds her head in his arms in a memorable scene. After her remains are buried she is remembered by Akira and is alive in Violence Jack.
(Source: Go Nagai wikia)