Natsuo Todoroki/Synopsis

History
Deemed failures by their father, Natsuo and his siblings, Fuyumi Todoroki and Toya Todoroki, were raised separately from Shoto Todoroki.

Natsuo had a very close relationship with his older brother, Toya before his death. Since then, he held nothing but resentment to his father. As they reached adulthood, Fuyumi convinced her brother, she could handle the family's circumstances and he should go out to pursue his own goals. So Natsuo left the household to study at college for a medical welfare degree.

Pro Hero Arc
Natsuo and Fuyumi Todoroki visit their mother in the hospital. He tells his mother that Endeavor was promoted to No. 1 and that he still cannot forgive his father for how Enji treated the family. He is surprised when his mother mentions the latter visits her and left her flower. He is told of his father is dealing with an internal problem.

While leaving the asylum with Fuyumi, the siblings watched a news report of their father dealing with a special Nomu called Hood. He was horrified to watch as his father was overwhelmed by a villain's attack. Natsuo continued to watch the broadcast wondering why Endeavor continued fighting. He frantically stated that he should just give up and retreat-like he did with the children he had. Fuyumi told him their father is the person who is too stubborn to ever do so. Not without great difficulties and ended up seriously injured, Endeavor manages to defeat Hood.

At his family home, Natsuo eats dinner with his sister and brother Shoto before their father arrives. Seeing him, Natsuo glares at him, especially at his scar, before Fuyumi tells them to at least put in the effort to talk. However, Natsuo gets up to leave as Enji tells Natsuo to speak up and the latter confronts his father on everything he's done, even blaming him for what happened to his brother Toya. Natsuo continues his rant about not knowing much about his brother Shoto because of him and ending it by saying he's incapable of change, only to be surprised by Enji stating he will face his past to atone before Natsuo leaves.

Endeavor Agency Arc
Natsuo is present at the dinner, Fuyumi held for Shoto and company. When Izuku Midoriya praises the food Fuyumi made, Natsuo says that Fuyumi has been doing all the cooking ever since their maid hurt her back and retired. Fuyumi then revealed that Natsuo also did some cooking as well, which surprised Shoto. Though Natsuo then states that since he over-seasoned the food, Endeavor made him stop, though Endeavor apologizes for not knowing prior.

After Natsuo finishes, he quickly leaves, still unable to sit near his father. While the others were cleaning up, Izuku asks Katsuki if he knew about Shoto's past, to which he reveals he was listening when the two of them were talking during the U.A. Sports Festival. While getting ready to leave, Natsuo overheard Izuku tell Shoto that he feels he's getting ready to forgive Endeavor, as he feels that if Shoto truly hated him, it would be fine to not forgive him, but Izuku voices his thoughts on that Shoto is a very kind person waiting for the right moment.

While outside, Natsuo is captured by the villain Ending. Natsuo is brought to the roads where Ending challenges Endeavor for a previous loss at his hand or else his son dies.

Held in the grasp of Ending, Natsuo is used as a shield by the villain, which halts Endeavor from attacking without hurting Natsuo. Soon, Natsuo is liberated by Katsuki Bakugo who safeguards him while Izuku and Shoto defeat Ending. Natsuo is then embraced by his worried father.

Dazed from the attack, Natsuo is asked by his father if he's hurt, while Natsuo only mentions how hot Endeavor's grasp is. Natsuo soon breaks free of Endeavor's grasp, as Endeavor reveals that he stopped to think in that moment fearing that if he were to save him, then Natsuo would never speak to him again. Natsuo hears his father confessed that he never meant to neglect him and Fuyumi all those years, but acknowledged that he failed to take responsibility by running away from them. He also acknowledges that he feels it's his own fault for Toya's fate. Natsuo tells his father he doesn't care, for he heard everything from Toya growing up and declares he will never forgive Endeavor because he doesn't see himself as kind as Shoto or Fuyumi.

Natsuo hears Endeavor state he still came out for Fuyumi and Rei's sakes and that he was trying to respect the feelings of his sister who is trying so hard to fix the family that Endeavor broke, signifying that Endeavor sees Natsuo as kind enough. Endeavor then states that it's okay if Natsuo doesn’t forgive him because he's not looking for it, he just wants to atone for his mistakes. Breaking into tears, Natsuo tells him that Fuyumi had been so happy lately, but every time he looks at him, he thinks of everything in the past. He questions why he has to be the only one to try and proactively change and what exactly will Endeavor do to atone.

After Ending is detained, Natsuo thanks Izuku and Katsuki, he asks Katsuki what his Hero Name is. Katsuki tells him that he picked it but won't reveal it to them.

After returning home with his father, Natsuo is asked by a frantic Fuyumi if he was okay after being attacked, to which he confirms because Shoto and his friends saved him. He soon listens as Endeavor speaks to Fuyumi about the home she is trying to build for their mother to return and how all he could do was rely on her. He then declares that he will have a new house built in a more convenient location for her and Natsuo where she can welcome their mother back to. When Fuyumi questions what he'll do, Endeavor reveals he intends to stay at the abode and live alone, as he realizes that the only thing he can do to help his family to take himself out of the equation.