Although Kotarou desires to be friends, Yoshino thinks the opposite and disagrees only seeing rivalry between the two. Behind all that, Yoshino actually looks up to him in a certain way. One attribute they both is share is they are loners, Yoshino at a higher degree, but they often have clashing ideas, so they are like opposites. Kotarou and Yoshino bicker and have quarrels often, and aren't reluctant to physically fight each other due to a former debate.
The two are actually close but it does not seem that way, with them both insulting each other and fighting for Chihaya's affections. Sakuya enjoys humiliating him with the nickname 'Fatarou kun'. Sometimes Kotarou sympathizes for Sakuya when Chihaya unintentionally upsets him with a comment with an agitated tone to it, then Sakuya turns solid stiff and paralyzed, frozen in shock. Kotarou uses Sakuya's phobia of cats to his benefit and makes fun of it, to Sakuya's displeasure.
The two of them are childhood friends, though the memories are hazy for Kotarou, because of the accident. It is revealed in the Terra Route that their relationship is nothing alike what it is in the Common Route. They practically despised each other, despite their similarities, though they sort of shared an older brother little sister bond. Except for that particular arc of the game, they get along suprisingly well, being classmates. But Kotarou sees Kotori as a potential love interest, no matter how many times Kotori rejects him, he doesn't desist liking her. It is may not actually be unrequited love, it is likely she likes him as well.
Lucia finds Kotarou a nuiscance at the beginning, finding it neccassary to make Kotarou compensate by giving him pain. Eventually, she resolves never to do that again, The two get along very well after the occured event in the chapel, where Kotarou vows never to leave Lucia's side.
Chihaya has a habit of saying "Thats what I hate about you!" Which at some point hurt Kotarous feelings but he decides to hide this. At first, Chihaya tends to have a tsundere like personality, but starts to warm up to Kotarou at a quicker rate than Lucia and Kotori.
Akane generally shows her other personality to Kotarou, which is not her myserious one. Akane has had a rough past, living in an orphanage specially for children with mental difficulties, so sometimes she needs someone to let out all her emotions to, which is Kotarou. Despite the fact she states she finds humans disgusting, Kotarou is an exception.
Kotarou finds it entertaining to tease and joke around with Shizuru, who does not realize his true intentions. Slowly they both begin to see each other in a more romantic view. At some point in her route, she reveals her past to Kotarou who sympathizes with her, and makes him love her more other than pity.