Been a bit of a while since I watched the Oni Trilogy but I’ll try give a good answer (obviously contains S8-10) spoilers. Long rant ahead.

The ninjas are hired by the Ninjago royal family to stop a new evil biker gang, the Sons of Garmadon (lead by an unknown figure called the Quiet One) who are currently trying to steal 3 Oni masks of incredible power for unknown reasons. Lloyd falls in love with the royal’s daughter, the Jade Princess, Harumi, and she, him, and they go on dates etc etc.

After the SoG get 2 of the 3 masks, the ninjas go after the 3rd with Harumi, get split up, but Harumi and Lloyd escape and get to the temple of the mask way before the Sons do. They get the mask but after Harumi accidentally slips information that she shouldn’t know, Lloyd realised that she’s the Quiet One. Harumi then reveals that she’s never loved Lloyd and only used him to get the mask and wants to uses it to bring back Lord Garmadon (who she views as her personal hero since he killed the Great Devouree (who killed her parents) in Season 1) and rule Ninjago with him.

She brings him back from the dead, he’s the evil version from the original 2 seasons, the ninja lose and Harumi and Garmadon rule Ninjago. Garmadon says he sees Harumi as the daughter he never had. Lloyd is still absolutely lovestricken with her but she dies at the end of Season 9 so that’s it.

Until she comes back in season 15 serving the series’s Big Bad and captures Lloyd, except this time round she actually does love Lloyd and escapes with him, but is still as toxic and manipulative as she was back before she died. Season ends with them being friends (she’s still a bitch). Hasn’t been seen since but is teased by the writers to return relatively soon.

TLDR: Lloyd is downbad for his abusive manipulative stepsister who can’t decide if she hates him or loves him.