These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Kaji: He has it just as bad as Hino. While in most stories, he's not portrayed as a jerk that just wants Hino for sex, but in the stories where Hino cheats on whomever she's with to be with him, he's usually portrayed as a jerk jock instead of the considerate guy that generally cares for most of his rivals (except for Yunoki).
Ensemble Darkhorse: Kazuki is generally well liked in the fandom for his upbeat personality and not being either a jerk or possessive toward Kahoko at one point.
Kazuki/Kahoko is surprisingly popular along with the Official Couple: Len/Kahoko. This might have to do with the fact that Kazuki is the first guy that falls for Kahoko and openly admits it relatively early. He and Kahoko also hang out relatively often without Kahoko getting nervous around him like she does around the other guys and in the anime are the first to get awkward moments with the Almost Kiss scene coming to mind.
Shimizu and Fuyuumi mainly for being around the same age and being paired up often in the series.
Shipping Goggles: Amou would happily ship Hino with any guy, and she even says how lucky Hino is to have danced with "the three hottest guys" — Shimizu, Aoi, and Kazuki — during the Cultural Festival (though, unknowing to her, Hino also danced with Yunoki). More recently, Amou lets it slip to Tsuchira that she thinks Tsukimori might be Hino's "main choice."
Kahoko at certain points in both the anime and the manga mainly during the third performance.
Hihara when going through his Break the Cutie phase around the same time as Kahoko ironically.
Fuyuumi as well, who gets picked on by her upperclassmen who are jealous of her playing skills. It doesn't help that she doesn't rank high in the competitions, and some random guys tell Shimizu and her that the freshmen should drop out if they suck so much.