Onimomogatari

Once upon a time, Momotaro, a boy born from a peach, defeated the demon of Onigashima. However, the bloodline didn't die with the demon... Amaro, a descendant of Onigashima's demon, just wanted to fit in with his tribe. One day, Sumomo, a descendant of Momotaro, appeared and told him that she will help Amaro grow into a great demon...

(Source: Kadokawa, translated)