This is just a hunch but I think that the future captain is in love with her og self or he falls in love for her new body and she's the one who teaches him to fight an such but the body may have a time limit or she goes missing and he finds out that they share a soul if that makes sense.
Comment by Rimuru Tempest on Hey, Little Duke, Just Trust this Sister! - Chapter 10
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Seems like it. There most liley is a time thing where the reason why he's so strong is becuase she taught him in the past. And since he's this frantic to find her in the original timeline it does seem like he loves her