At some point in her life, Rei married Enji Todoroki, who used his power and influence to force her family to agree to the marriage, so that he could use his and her Quirk to produce a strong offspring. She appeared to be unhappy in this union, as her son would have memories of her crying very often.
When Endeavor began to brutally train Shoto when he was only five, she would attempt to intervene, but was physically assaulted as a result. Despite this, she always supported Shoto in these dark times. When Shoto expressed his worry over someday becoming a man like his father, she assured him that he can become whatever he wants to be, and that he is not bound to his bloodline.
However, Rei started to lose her sanity due to all the abuse received from her husband, finding it painful to look at Shoto because his left side reminded her of Endeavor. One day, as she talked with her own mother over the phone about her trauma and recognized that she no longer had the mental stability to raise her son, she was startled by Shoto's sudden appearance at the door. Seeing Shoto's left side, she snapped, and threw boiling water from a nearby kettle over the boy's face, causing a burn over his left eye, even though she immediately regretted doing so and, in tears, tried to heal him using her Ice powers.
However, Shoto bore no ill will against her for this incident, instead directing it at his father, whom he blamed for both his and his mother's suffering. Some time after the incident, Rei was admitted to a psychiatric ward.
The day after the U.A. Sports Festival, Shoto visits his mother in the hospital. Rei is staring out the window and turns around when he enters. During Shoto's visit, she apologized through tears to him, and smiled as Shoto told her he bore no ill will against her and knew she did not mean to harm him and he forgave her a long time ago. She then told him that she wanted him to be happy and save people, which allowed Shoto to finally leave his past behind.
Her children Fuyumi and Natsuo visit Rei in the hospital. Smiling, Rei shows off the letters Shoto has sent her. Natsuo tells her that Endeavor was promoted to No. 1 Hero, and Rei reveals that Enji has visited repeatedly but they have not faced each other.