Anime

Chloe was an incomplete summon like the other kids. She shows up long after Leon (I forget but I think he just gets isekaid like Hinata). Like she said to Hinata her future powers came to her.

From what I recall Chloe and Leon are not actually related but she looked up to him and he's a huge siscon which is why he's been looking for her so intensely.

There is some bootstrap paradox but we never get into that in the story. Chloe has made an unknown amount of loops and only remembers the most recent loop in addition to her "current" loop. IE right now she has memories of the last loop (which we saw as werid flashbacks with Rimuru dying) and the one where Hinata dies while Rimuru is still alive (the one we watched in the actual anime). To spoil a bit the goal right now for Chloe is to do the same things she did last loop so history is the same leading up to Hinatas death.

Some spoilers about how the Tensura world works. So basically the world Rimuru is currently in is the "Central World" the first world Veldanava created. Yes first, Veldanava created many worlds, such as Rimurus original world and many more. All of these worlds exist in the same universe, at the same time but time is not constant. IE some worlds are "in the future" and other worlds are "in the past". Chloes power is strictly jumping backwards (involuntarily when Hinata dies) on the "global timline" that is to say no new reality is created or hopped to.

We don't know where the time travel power initially comes from but it's likely Chloe herself created it after Hinata died the first time as it's a Unique skill. The real bootstrap paradox imo is the mask since Chloe got it from Rimuru who gets it from Shizu who gets it from Chloe... also worth mentioning that Rimuru perfectly replicated it which is insane and is what got Diablo to become Rimurus subordinate