Unconfirmed spoilers, but same source that got info right for the last couple chapters
We don't get much about Toga's past besides the Deku lookalike was named Saitou, she drank his blood, and killed birds as a kid
Toga kills Kizuki by turning into Ochako and using Ochako's quirk to make Kizuki float high, she apparently can also duplicate quirks
Questionable at best