3 ideas
Keiji Kiriya, a raw recruit in the United Defense Force, is killed on his first day fighting the alien Mimics on Kotoiushi island off Japan. The loop ends only when the Mimics' loop-generating server is destroyed, and because both Keiji and Rita are tied into it, one of them has to die, so Keiji kills Rita to end it.
Each loop restores Keiji's body to its untrained state, so strength and conditioning never accumulate. Only what he has encoded as knowledge carries over: perception, timing, decisions, and memorized sequences of events. Mastery under these conditions means turning physical skill into cognitive skill, such as knowing where to look, when to move, and what comes next, rather than building capacity.
Because nobody else remembers the previous loops, Keiji's hard-won knowledge and relationships reset for everyone except him. The gap between what he has lived through and what others know grows with each loop, until his competence seems uncanny and his inner life is unreachable. The only person who can understand him is Rita, the one other person who has looped, and that shared experience is the same link that forces one of them to die.