I think the manga/novel is jp. Guy reincarnates into "third-rate villian character" in a dungeon school type setting. There's 3 or 4 other reincarnators. He uses cheats to gain levels and also brings his sister and parents into the dungeon so they level and don't get killed by the story. He tries to take a bystander role with story. Parents own an equipment shop? He's seen as lazy, fat, trash. There's a demon guy from a higher floor that is also a reincarnator and is a fan of one of the novel characters, mc's little sister or sisters friend?

TYIA

Nevermind, found it was Evil Avalon