I was curious, are there any stories where someone goes back in time (deliberately or otherwise) on someone else's account? Particularly to save them or help them out of a bad situation, though I may be fine with ones where they go back for a chance at romance or something as long as the person ends up pretty quickly focusing instead on protecting that person or helping them upon realizing that they're dealing with various things. Closest I can think is Erased, though that's more general in his attempt to save a group of people and prevent a series of crimes, rather than him being entirely focused from start to finish on a single person.

Steins Gate but I recommend the anime
Psyren
A Returner's Magic Should Be Special

Orange (TAKANO Ichigo)

Tokyo Revengers 🙂 that's the first manga I remembered

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Re:Marina

The God of Calamity Wishes for Naught