OVA 9

Genos and Memory Loss (ジェノスと記憶喪失, Jenosu to Kiokusōshitsu) is the ninth OVA of the One-Punch Man anime series.

Summary
As Genos and Saitama watch TV, Saitama accidentally knocks Genos out trying to reach his potato chips and Genos wakes up with memory loss. Genos reads some manga left on the floor in Saitama's apartment, and picks up various character personalities. For example, one personality is childish, talking in a high-pitched voice. Another is a delinquent personality when Genos insults Saitama after discovering Saitama is his master. Genos also tries to flirt with Fubuki. Before running away to try remember who he is, Fubuki tries to take advantage of the situation, trying to get Genos to join her group.

Genos encounters a monster known as the Lord of Mountains. Genos leads the monster away from civilians as he fights the monster. As his memories have been wiped and he has forgotten how to fight, he is beaten down, until the Lord of Mountains shocks Genos with electricity. This gives Genos a memory reboot, he then goes to fight the monster like normal. Killing the Lord of Mountains with a single incineration blast, Genos returns to Saitama and apologizes for the trouble he has caused.