revelations
I've already said that the library conversation that the two of them had in volume 15 was the first step between their final reconciliation. And it was. Because after that, Yukino was able to see a very important part of Arima's true self. She 'heard the emotion behind his words' and because she knew the real Arima somewhat, she was able to finally pinpoint exactly what was wrong, something that Arima himself didn't even realize. The difficult part was getting Arima to listen to her. After Arima 'punished himself' for violating Yukino by seriously injuring his hand (it honestly made me wince!), Yukino saw his bloody hand and was so angered that she slapped his face, and then hit her hand against a window and wounded herself to prove a point before saying that she didn't consider what he'd done to be rape because she'd never told him 'no'.
After carefully considering all that he'd told her over the years, she came to a simple conclusion: If you were hurt because you weren't loved, I think your emotions would've been less intense. Arima... Maybe all this time... You just wanted to love someone. As corny as that line might be, what Yukino was trying to say that deep down, Arima wasn't afraid that she didn't love him anymore, that his new parents didn't love him anymore. What he was afraid of was that his love would be trampled on. He had trusted and loved his birth mother so wholeheartedly and in the end she had seen him as nothing more than an annoyance. So in the back of his mind, Arima was afraid to give himself wholly to another person because he didn't want to be betrayed again.Once both of them realized this, the walls that divided them finally came down completely. For the first time in their relationship, there was nothing kept between them and they were able to truly understand and love one another. Arima finally realized that Yukino really would accept him, no matter how dark his true personality may be. Ironically, after this revelation, the dark tinges to his real self begin to fade and Arima is able to really become happy.