Love interest: Makima x Quanxi x Yasaka x Grayfia x whoever
Can be a reincarnated Mc but she must either be a replacement or related co Mc of Issei and Yuji she must replace Denji as the Chainsaw devil though
No system unless it's only a skill panel aka a mastery panel must be in universe powers only. She doesn't join Rias or if she does it's only for till the marriage fight then she has to be let go cause Rias isn't strong enough to hold her this is when she is found by Makima who produces a contract between her and our MC's deceased parents or if she is the sister to the original Mc that claim's the oldest child aka Mc as hers maybe one of the parents were Devil Hunters or informants for the supernatural or curses you can decide how the contract happened but it must happen. The Devil Hunters have a division for the supernatural this includes the JJK side of things as well.