In a monologue around the halfway point, Momo describes his girlfriend as cute, smart, kind, hardworking…and a little weird. That last adjective sets her apart from other cute/smart/kind girls, some of which Momo knows, and one of which (Rio) harbors unrequited feelings.
And Momo likes the weird. She may like collecting his “used stuff”, but he’s always thinking about her in clothes he sees on TV; and is only too happy to get an indirect kiss from the teddy bear he won for her. Her weirdness helps him realize he’s a little weird too.
The only things that frustrate him are not knowing where Yuki gets information on him he himself did not provide. It’s a secret she possesses, which means it’s distance she’s keeping from him.
Mind you, all healthy relationships have a measure of distance – you can’t be with someone every waking moment – and this is hardly a major crisis for Momo. He’s just…curious. And yes, tartar sauce on omelette rice is perhaps the weirdest part of him revealed thus far!
Momo is also an easy-to-read, jealous fellow who is very possessive of Yuki. Under normal circumstances this would be distasteful, but Yuki happens to love his fierce protective nature, even when he slaps away the hand of Seiichirou (Ikue’s brother), who happens to be Yuki’s “Momoformant”.
It demonstrates he thinks and cares about her every bit as much as she thinks and cares about him, which makes her very happy. Neither of these weirdos could be good matches for just anyone, but they seem to be perfect matches for each other, which is why it’s so easy to root for them.