Kawabata was a student and good friend of Misaki's grandfather, cheerful and kind. He's good at math and science, and helps tutor Misaki in math. When Misaki's grandfather dies, he assumes all responsibility for Misaki, and tells people Misaki is like a younger brother to him. In reality, though, he held romantic feelings for Misaki, and their relationship was not platonic. He's the one who gave Misaki his flower necklace, as a gesture to show he's not alone.
He severed his relationship with Misaki and got himself a girlfriend suddenly, and started behaving very possessively and cruelly towards him afterwards, calling him all the time and meddling in his life while holding him at an arm's length. Kawabata almost chokes him when he yanks the flower necklace off Misaki's neck, which traumatizes him. He taunts Misaki about his sexuality and his feelings for Arikawa, and is very displeased when the two get together. He tries to convince Misaki they won't work out, but Misaki stands his ground.
It's revealed Kawabata started dating a woman out of a desire to be socially accepted; from a young age he was pressured to do well in school and be successful, and he knew a relationship with an underaged boy would hold him back. After a last-ditch attempt to win back Misaki in Chapter 11, however, he cancels his wedding preparations with his girlfriend, telling his family he can "think for himself" now.
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