Shortly after Ranma and Genma abandoned Ukyo, she met a young man named Hayato. Hayato and Ukyo fought over whose specialty food was better (her okonomiyaki versus his takoyaki). The loser would be forced to wear a mask forever. Ukyo won the contest, and Hayato remained true to his word- he has worn an octopus mask all his life.
Six years later, Hayato and his pet octopus Patorashu track Ukyo down and try to exact revenge. They manage to disrupt business at Okonomiyaki Ucchan's for over a month before she is able to find them and challenge Hayato once again.
After awhile, Ranma takes it upon himself to find out what Hayato looks like (Ranma believes there must be something unique about his face). Ranma manages to remove the mask, but it angers Hayato so much that he adheres a mask to Ranma's face as punishment.
The Meaning Behind the Name
Hayato means "quick man" while Myojin means "strange or unusual man".