Rei Saiba is Neo Saiba's sister who was pulled into the Digital World with him and the Alias III members Sigma, Mari Goutokuji, and Hideto Fujimoto.

Taichi Yagami and Zeromaru first meet her when two Flymon were chasing her and the Flymon were killed by Zeromaru. Though at first she is scared of Zero, after he saves her, she warms up to him when he saved her. She cannot walk in the Real World due to a recent accident, but she is able to walk in the Digital World and her pendant turns out to be a Digimental that Neo uses to make Arkadimon stronger.

She gets mad at Neo when she finds out that he wants Taichi and Co. out of the way. He has Rei locked away in a room, but she is freed by Pal and Pul. She, Sigma, and Mari escape from Daemon's castle. When it came to the final battle between Taichi and Neo, Rei attempted to commit suicide via jumping off a cliff created by Arkadimon's God Matrix. and is saved by Zeromaru. Neo stops the fight after hearing Rei's plea. When returned to the Real World, she keeps up her friendship with Taichi and the others.