The official name of the work is labeled 恋ではなく ―― It’s not love，but so where near. As mentioned, more romantically, it’s best changed to “It’s not love, but something close.”
To put blatantly, at its core, it’s still love. The protagonist and the heroine’s relationship is best described as being adversarial. When the heroine was questioned about her feelings towards “Acchan”, the subject of the first route, she attributed numerous things to him on numerous occasions. He was a subject of yearning — someone to impress, and someone to aspire to. In the true route, when she was asked about what she thought of “Acchan”, she described him to be a close friend, a confidant, a “true friend” in literal translation. When she was asked about her relationship to Norifumi, the protagonist of the work, she stated that he was an enemy, someone to compete with.
Every since they were children, they wholeheartedly competed against the other, for the sake of winning Acchan’s affections. As Yumi put it, they competed to be his ‘best’ follower. Acchan was best described as being the jack-of-all-trades, wholeheartedly pursuing whichever interested him. As a result, Yumi incessantly competed with Norifumi. In the true route, when Acchan had asked why Yumi didn’t love him romantically, she had told him something along the lines of “because you treated me specially.” Acchan had, with an air of seniority, wanted to protect Yumi, since she was the girl of the group, and someone he was romantically interested in.
Norifumi on the other hand, didn’t care. He had at a point, picked up photography as a means to compete, and Yumi, subsequently entered the world of modeling as a means to ‘not lose to him.’ The dynamics of their relationship is special, in that they only care to ‘not lose to the other.’ Yumi, after she had ascertained that Acchan gave her special treatment, then asked how he would have treated her, were she born a man. Acchan replied something along the lines of “just like I would have treated Norifumi.” In other words, since Norifumi doesn’t give Yumi any special treatment, because they’re tied to a basis blind to sex, looks, or any other distinction other than the will to compete, she had fallen in ‘love’ with him. It’s a love built on the basis of passion, a love which surpasses the physical.
So, in a sense, the mutual inferiority complex was necessary. They were both driven by the will to not get left behind, to not lose, that they respectively aspired for the best. As Norifumi got better at photography, Yumi aspired to become a better model. In a sense, the dynamic encouraged the two to motivate the other. You could argue that Norifumi’s a hetare on the topic of love because he doesn’t want to lose to Yumi, so he doesn’t pay attention to anything other than photography. As it was explicitly written, when Norifumi’s in the midst of filming, he’s like a different person — an individual with presence, someone notable.
So, it’s not love, but something close. It’s a love which transcends gender in a way. It’s a type of pure love.