Rei Todoroki/Synopsis

History
Rei born in the Hiruma family. When she was a young woman, Enji Todoroki proposed to her family to marry her. The family expressed great joy and honor in being reached out to by the man who was said to be the next candidate for Number 1 hero. Even knowing that this marriage was a Quirk Marriage, Rei still agreed to it for her family's sake. Rei showed a cold and distant demeanor, but when they were walking through the garden, Enji noticed that she was observing some flowers in particular, and asked her if she liked them, to which she answered yes.

Enji sought with that marriage to have a son who would inherit both his hellflame Quirk and Rei's Ice Quirk. Their first son, Toya Todoroki, did not meet those requirements, but was born with a firepower greater than his own, so Enji decided to train him who did not meet expectations. Even so, he still sought a child with Half-Cold Half-Hot, and Rei wanted more children, believing they could encourage each other, which ultimately led to Fuyumi's birth, who was also not born with the desired Quirk.

Despite neither child bore the ideal quirk Enji was hoping for, there were no problems in the family situation, as Enji was happy with the life he had, training Toya due to his potential. However, the situation changes when the hair of their firstborn begins to bleach and his own Quirk burns his body. Rei and Enji went to the hospital to find out what was happening to him. They were informed that Toya takes after her mother physically, which means his body is meant to withstand freezing temperature and not the flames of his powerful fire-based Quirk.

Because of this, Enji suddenly stopped training his son Toya, trying to dissuade him from becoming a hero, explaining the reasons why. Unfortunately, Toya refused to do so, insisting his father train him, training on his own every time his father refused, getting more and more burns. As if this wasn't enough, Enji decided to keep trying to have a child who would inherit the two Quirks. Rei told Enji that the choice he wants to make is too cruel, especially since Toya knows what he wants from his children. Enji tells her that no matter what he says as Toya inherited his stubbornness. He tells her it is the only way to make him give up because he will never be able to surpass All Might.

This would ultimately lead to Natsuo's conception, which also did not meet their requirements. This whole situation would begin to affect Rei psychologically, starting to look paranoidly at Toya, who was also started falling into despair. Several years later Shoto was born, finally having a child with the Half-Cold Half-Hot Quirk that Enji was so desperately trying to conceive.

Toya refused to accept that his youngest brother would be the one to surpass All Might, so he continued training his Quirk more intensely without caring the burns. This caused him and his father to have an argument at home, which Rei witnessed while carrying an infant Shoto. Enji asked his son to give up his dreams once and for all, telling him that there are more things beyond the world of heroes, with Toya replying in despair he cannot as he was the one who lit those ambitions in him. He tells him to look at him as he charges with his flames activated at his mother in order to attack Shoto.

When Enji began to ruthlessly train Shoto when he was five, she would attempt to intervene, but was physically assaulted as a result. Despite this, she supported her son and encouraged him to push through the hard times. When Shoto expressed his worry over someday becoming a man like his father, she assured him that he could become whatever he wanted to be and that he was not bound to his bloodline.

At some point, Rei began to lose her sanity because of Endeavor's abuse, finding it painful to look at Shoto because his left side reminded her of Endeavor. One day, while she was talking with her mother over the phone and discussing that she no longer had the stability to raise her son, she was startled by Shoto's sudden appearance at the door. Seeing his crimson hair, she snapped and poured boiling water from a nearby kettle over his head, severely burning his face. She immediately snapped back to reality and tearfully apologized, using her ice Quirk to soothe the wound.

Shoto bore no ill will over this incident, instead directing it at his father, whom he blamed for his mother's suffering. Sometime after the incident, Rei was admitted to a psychiatric ward.

U.A. Sports Festival Arc
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.

Vs. Hero Killer Arc
Shoto continued to visit Rei on his days off every Sunday. On one Friday after school, after learning about the upcoming Parents' Day, he went to go visit her, thinking about the awkwardness over wanting to ask her about it, even though he already knows the answer. They're slowly able to communicate with each other, with Shoto talking about his recent experiences at school, when Fuyumi shows up, bringing her mom's washed clothes. She finds Shoto's Parents' Day request form, leading Rei to sullenly apologize that she can't come, disappointing Shoto. Fuyumi steps in to ease the tension, saying that she'll go in her place, even suggesting recording it for her to see, which slightly embarrasses Shoto, but he eases once he sees her improved reaction.

Pro Hero Arc
Her children Fuyumi and Natsuo visit Rei in the hospital. Smiling, Rei shows off the letters Shoto has sent her. Natsuo then tells her that Endeavor was promoted to No. 1 Hero, and Rei reveals that Enji has visited repeatedly but they have not physically faced each other.

Endeavor Agency Arc
On New Year's Eve, Shoto once again goes to visit Rei, where they have lunch at the hospital cafeteria. He reflects on how it was the first in a long time since they've been able to eat together like that, noticing her steady improvement. He also tells her about finally getting his hero license, which she is incredibly happy and proud to hear about.

Sometime afterwards, Shoto receives a letter from her, hoping her son won't catch a cold in the upcoming chilly weather, saying how she's been going on more outings lately, and that the doctors' told her she could be discharged soon, thinking about wanting to live with her children again, to make up for past mistakes. Later, Enji reveals to Fuyumi and Natsuo his plan to help get a new home for them and their mother to live in together, knowing all the work Fuyumi has done to try and patch things up, while he stays at their old place to give them space.

Paranormal Liberation War Arc
In her room at the doctor's, Rei is watching the news of Gigantomachia's rampage when the image starts to change and the video feed is replaced with an image of Dabi. Along with the rest of the world, she watches in horror as he reveals that he is actually Toya Todoroki, her eldest son, and has become a villain who has killed more than thirty innocent people.

After hearing the news, Rei is released from the mental hospital and joins up with Fuyumi and Natsuo to meet Enji and Shoto in the hospital. As Shoto's classmates are consoling him, Natsuo and Fuyumi arrive while she appears in the back, slightly surprising Shoto.

The family then goes to meet Enji, who apologizes to all of them while expressing the pain in his heart. But before he can finish his sentence, Rei interrupts him and asks him about the pain in his heart, reminding him that the rest of his family have held the same regrets and guilt in their hearts for much more and much longer than he has. Enji is completely shocked at her arrival and asks what she is doing there. Rei replies that she is there to talk about their family and about Toya while carrying the flower Enji had given her before in her hand.