Okay, before the prologue, Kuro confesses to Maru. Maru rejects her because he likes Shiro. He and Shiro start talking to each other after school at the riverbank because Maru likes her writing and she was trying to get his attention. The rumor about Shiro dating Abe starts making the rounds at school and Maru gets depressed. Kuro, taking advantage of Maru's mental state, convinces him to try and get revenge on Shiro at the Confession Festival by humiliating her. At the same time, Shiro is plotting the exact same thing against Maru because she was still mad that Maru stopped coming over to her house back when he was a child actor(she had short hair at the time and Maru thought she was a boy). Shiro and Abe are faking the relationship to goad Maru into action(Shiro, for revenge, and Abe, to see Maru act again as he's a huge fan).
As Maru was trying to practice acting again, he has a bad episode of PTSD and passes out. He wakes up in the infirmary and talks to Shiro, telling her why he quit and disappeared(during a drama he was working on, his stuntwoman mother, who was playing as the mother of his character, got hit by a car in a legit accident and died, which traumatized him because he witnessed it). So, Shiro finally knows the truth and that he also remembered the promise he made to her to act in something she wrote and decided to give up on her revenge plot. Fast forward to the Confession Festival, Abe goads Maru some more during his set by doing a song and dance from a drama Maru performed in, making Maru come out on-stage to show he's still got it.
Maru fakes out Shiro and confesses to Kuro, who then flat-out rejects him, completing *her* revenge on Maru for rejecting her(yes, she set eveything in motion just for this moment). Of course, she doesn't consider the consequences of her actions and the fact that she "won" by making him legit fall for her, only to piss it away because she was petty.