As Momo was considered quirkless everyone in 1-A abused her even her parents neglected her and hated her(not Loid and Yor). Only one person didn't hate her. Him and his family were the only ones who loves her and one day when she ran away everyone was sad  that she ran away but she kept a picture of him and her as her love grew bigger for him as he was the only one who didn't bully her as he always protects her. That same day she ran into a women who was the previous Seraph as she saw that Momo was hurt and she took her in and raised her as if Momo was her own daughter and taught her to be strict when it came to her foes as Momo learned fast and became the next Seraph as she became a deadly assassin force. Years goes by Momo was known to be the deadliest Seraph based on how she felt until she met her lover who doesn't want nothing to do with her past.

Ship

Izuku (merchant) x Momo(Seraph)

Once she heard about Izuku being a merchant despise having a quirk she immediately began looking for him and she found him after. She learned after she ran away he doesn't want to be a hero as he also found himself being an assassin which made her fall in love with him more. Also after Momo ran away the girls tried to persuade their love for Izuku as they knew that Momo loved him so they could make Momo feel alone even more as they actually hate her as the boys wanted her for her body when she got older and Izuku rejected them which made them have a fight and a brake off of friendship between him and them.