Episode 1

Senshi was the adopted son of Sirzech Lucifer and Grayfia Lucifuge and was born "Weak" and they neglected and forgot about him and he was also a member of Rias Gremory's peerage but they too have forgot about him.

But he wasn't actually weak, Its quite the opposite for he is a Saiyan and he has been hiding his power because he believes it will corrupt him, But after being forgotten for so long he refuses to hide it anymore and embraces his inner warrior

(Non of the pictures,anime,music,videos are mine i only own the story)