Actor: Satoshi Matsuda
And one day when he picked her up from work for lunch, after she issued a light-hearted scolding to him for fighting again (bruises on his face, a dead giveaway), they went to the beach and she played in the sand. Then she came to him and told him the experiments they were doing were becoming frightening. She wanted him to stick around, pick her up afterwards and hold her, be strong for her. So he waited.
Something happened, perhaps Eri screamed. Whatever it was, Ren tore into the lab to investigate, to find Eri leaping from a cushy black chair and a giant bat creature swooping down on her. Exactly what happened next is a little confused, but Shiro Kanzaki gave Ren a card deck, told him how to bind the monster, and probably told him the terms.
Ren fights to win so that he can use his wish to save Eri. She lies in a coma in the hospital, where he goes almost every day. The doctors and nurses know him well and keep him apprised of her condition. But it was slowly deteriorating. He wears her rings on a chain around his neck to always remind him. The pain had started to dull, until regaining his memory made it fresh and sharp. The tentative friendship he was forming with Shinji Kido suffered for this, for Ren knows for him to win, Shinji has to lose... to die. Shinji and Yui both influence him to gentler emotions, and he was starting to break under the strain of what he had to do.
Shiro Kanzaki allowed or forced Eri to wake from her coma. Precisely why is uncertain, perhaps to give Ren hope that her life could be saved. It backfired. Ren fought the monsters with more determination than ever, and stopped hunting the other Riders. But the discovery that Yui was dying changed all that, for it was HER life he started to fight for. And while he would not kill to save her, it became clear he was willing to die. Ren became infected with Shinji's compassion, he began to care whether Kitaoka lived or died and checked on him. But when Shinji died it nearly broke Ren. Shinji gave up trying to save Yui because he knew she didn't want him to, and turned back to his wish to stop the fighting. And that giving up told Ren what he had to do. When he was the last, he brought his wish and the power to save Eri's life, but the injuries he'd suffered fighting Odin cost him. Though it is possible he was only unconscious, I believe he died.