The time travel mechanic is more realistic than the fact that the girl genuinely cares for the mc.
Nah. We see his character, and it's not bad at all. He's very intelligent, scoring near the top of his class even though he's currently being
MEGABULLIED. He's driven, being able to cast aside everything going on with his family and his life to focus on his dream of being a vet. He's empathetic, still caring about his mother even as she has totally lost it. He's brave, too: shielding her from his bully, saying that they can do whatever they want to him, but not to hurt her.
The thing about this story that's making it compelling to me is that Yotsuha looks and sounds like a moeblob, head empty, bright and cute girl, but she's
acting that way as part of a deliberate plan that she has thought deeply about, and seems to have done several times, finding failure points along the way. She has a very good reason to like the guy, and she's so attached to him because she has lost him already. Multiple times. She blames herself, partly; in her dream you see that her tears make him turn away and kill himself.
Her love is so fanatical, her joy so intense, her personality so vibrant because she has
lost him before. She's doing everything she can to avoid losing him again.