She's legit. Yeah, she, her boyfriend and some other boy were the ones caught with MC and Truck-kun.
The boyfriend is the cause of the warp incident, but he's not the one with the rewind ability. The rewind belonged to a Miko, who was enslaved working for royals in an unnamed country that ended up summoning boyfriend-kun like 100 years in the future. Boy meets girl, they get along; boy gets sent into war because that's what he was summoned for; boy gets a bad case of death because shock horror, isekai-ing doesn't magically give you special abilities in this world, and throwing a 16/17yo peacetime Japanese boy into a medieval war will have unsurprising results.
Miko gets a worse case of feelings because all she felt was despair before the boy gave her hope, uses all her ability to rewind the world to a state where the boy can be saved, the ability does this by trying to summon someone else with similar feelings into the world in the past.
The world resists because rewriting the past by inserting an outsider is nonsense bollocks, and nothing happens for a while, but MC alters the course of history by rewriting small events that end up being pretty major things. The world's resistance weakens until the summoning finally succeeds, causing the warp incident.
Orsted's time loop is an ability/curse placed on him by the first Dragon God, and he has technically also been sent into the future, because he was alive during the original Laplace campaign (the first Dragon God was his father). Orsted says that the Miko had a weak fate and died every loop he went through, and I think the implication is that the Miko remembers every death she had for each of Orsted's loops - and there were a lot of them, which led to her mental state before she met boyfriend-kun. Whether her ability to rewind and Orsted's loop are related is unexplored.