that woman
We never get all that many personal details about Arima's birth mother. Her surname is never even given, just her first name: Ryoko. She is a pathological liar who beat her own three-year-old son, despite the fact that Arima was tiny and never so much as talked back to her. She says in volume 14 that when she was 13, she was raped by her mother's second husband and that her mother blamed her for it instead of saving her. If this is true, then it would really explain her twisted personality, but there's a chance that she was lying about that, as well. The key point is that she doesn't want anyone else to be happy -- as if because she was denied happiness, no one else should have it, either, be it Arima or his father or the rest of their family. She beat Arima to the point where his face was completely swollen, bruised, and beaten up, she barely fed him so that he looked like one of those poor children from Africa, and she emotionally abused him by locking him up, isolating him, and talking down to him. There was literally never a time that she was kind to him in the few years that they lived together. Ryoko was even receiving child support money from Reiji, but she didn't spend a dime on Arima. In a particularly sad scene, it showed this woman trying to give Arima a free popsicle out of pity, but Ryoko only got angry and stomped on the popsicle, saying that her pathetic son 'didn't need it', then she took him home and beat him.
Arima clung to her. He said: To a child, the house is his world and his parents are god. Ryoko was certainly very beautiful, though scary and somewhat eerie, and he said that he was always chasing after his beautiful mother's back. She was his goddess. This only made it worse when she was horrible to him, but he was unable to ever stop believing in her. He kept waiting for the day that she'd smile at him truly and be kind to him. The bruises and emotional scars she gave him stuck with him for the rest of his life despite the fact that he sealed his memories of her away -- literally, he put them into a mental box and buried them deep. Still, he could never really love someone ever again for fear that they would betray his love as she had. Even though his new parents were very kind to him, Arima couldn't open up to them. He loved Yukino, but couldn't really open up to her, either, until much later. When he finally remembered all that his mother had done to him, it wasn't good, either.When Arima was shown on TV because of his win at the national youth kendo tournament, Ryoko took notice of him and came to bother him. The main reason was that she hoped to be able to get some money off of him (her company was apparently going out of business) but the second and equally important reason was that she simply wanted to bother him and keep him from being happy by reminding him of his roots that he'd always been ashamed of and tried to hide. There is a part towards the end of volume 14 where Arima fainted after being reminded of how much Ryoko beat him when he was a child and he thinks of Miyazawa, 'his goddess'.
This was particularly telling because it showed that slowly, his mind had shifted. His mother was no longer his goddess, Miyazawa was. The problem was that he had no idea how to deal with his mother and he couldn't tell Yukino about it, either, because he was afraid she would think less of him if she found out about his birth mother. He lied to Miyazawa and said that she was merely someone from the neighborhood who had known him as a child. This led to a lot of problems between the two of them because Yukino was keenly aware that he had lied to her.
Eventually, with the help of their friends, after the two reconciled their differences, they were able to drive off Arima's mother, at least in the short term. She would come back with full force later, but for a very different reason -- the return of Arima's birth father, Reiji, to Japan.
Sadly, Ryoko never really learns her lesson. She never apologizes or learns from her mistakes, and is never really punished. In fact, the person she hurt most forgave her and she merely ran away. However, for someone like her who really can never change, I don't think any other ending was possible.