Natsuo Todoroki/Relationships

Enji Todoroki
Natsuo feels a strong animosity towards his father, for his negligence towards him and his siblings who did not inherit a powerful Quirk to surpass All Might, as well as for being the reason that his mother ended up hospitalized, and considering him the principal responsible for the death of his brother Toya. Natsuo mentions that he hardly remembers his father, which means that Enji did not do much in raising him. This aversion does not allow him to spend much time in the same room as his father.

After Endeavor defeated Hood, while getting a scar on his face similar to Shoto's as a result, Enji returned home, finding his children eating dinner. Natsuo, along with Shoto, tensely glared at him and only spoke up about his new scar. When Natsuo moved to leave, Enji stopped him and told his son to speak up. With this, the latter angrily confronted him on everything he's done, even saying he's incapable of change and blames him for the distance between him and Shoto, as well as what happened to Toya. Enji simply remained silent at the rant before calmly stating he will face his past to atone. Natsuo was surprised by this statement and left. Enji spoke with Fuyumi and stated he chose the wrong words with Natsuo.

Months later, Fuyumi organizes a dinner in which she invites Natsuo to eat with his father, brother and Shoto's classmates, Izuku and Katsuki. Out of respect for his sister Natsuo accepts. The tension with Enji remains high, as he is the first to leave. Shortly after leaving dinner, Natsuo is captured by Ending, a criminal who wants Endeavor to kill him. By putting his child in danger, Endeavor freezes because he's traumatized by the look on Natsuo's face. Fortunately, Shoto, Izuku and Katsuki manage to defeat Ending and rescue Natsuo.

Endeavor embraces Natsuo, grateful that his son's life is safe. As Natsuo breaks free of Endeavor's grasp, Endeavor confessed that he is regretful for what he did to the family due to his negligence and the damage caused accordingly. Natsuo tells his father he doesn't care and declares he is unable to forgive him, unlike his mother, Shoto, and Fuyumi. Endeavor responds that he sees Natsuo as kind enough and 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.

Crying, Natsuo questions why he has to be the only one to try and proactively change and what exactly will Endeavor do to atone. Later, Enji confesses he will have a new house for him, Fuyumi and their mother, as he realizes that the only thing he can do to help his family is to take himself out of the equation.

When visiting his father at the hospital after his older brother's broadcast, Natsuo was initially cold and asked his father why he was crying. After reminiscing of Toya and the events that made him become a villain, Natsuo stated that Endeavor was the root cause for Toya's descent into villainy. However, he also says that he had a part to play as well, and believes if he'd been more open with his father about his mistreatment of Toya in the past, things wouldn't have turned out so badly, choosing not to place all the blame on his father's shoulders and instead offering to be there with the rest of the family to help his father stop Dabi.

Rei Todoroki
Along with Fuyumi and Shoto, Natsuo usually visits Rei in the hospital. Like his other siblings, Natsuo has a positive relationship with his mother, with Natsuo complimenting his mother's healthy complexion and recommending her to get a phone to better keep in contact with Shoto.

Natsuo loves his mother very much, and he cares for her, and that she has ended up in a hospital for several years is one of the reasons for Natsuo's resentment towards his father, whom he believes he never apologized for what he did. However, Rei reveals that that statement is not true, because his father did come and visits many times but not face to face. From this conversation, Natsuo understands that his mother has forgiven his father. He respects her decision even if he doesn't share the same will.

Toya Todoroki
In Fuyumi's words, Toya and Natsuo were extremely close to each other, to point of being inseparable as they always played together as kids. Toya would also tell Natsuo everything about his personal conflicts and inner struggles, leaning on him for emotional comfort. Though Natsuo did indeed love his brother and listened to him, when Toya was talking to his younger brother when he was sleepy, Natsuo asked Toya to talk to Fuyumi every once in a while, slightly exhausted at having to listen to his brother's problems so often. This would hurt Toya, who believed Natsuo was brushing him off just like their father did, and explained that Natsuo was the only one who could understand him and the only one Toya believed he could really talk to, demonstrating just how much Toya relied on his younger brother. His supposed death, which happened shortly after his mother went away, greatly affected Natsuo, which caused him to greatly resent his father, whom he considers to be primarily responsible for what happened to Toya.

Nowadays, Natsuo still remembers Toya and still can't forgive his father for it. When Dabi revealed himself in public in an attempt to attack Endeavor and Hawks, Natsuo appeared greatly shocked by this despite not knowing his true identity around that time. Around the time of the Paranormal Liberation War, Natsuo is shown to be deeply horrified upon learning that Dabi is actually his long-lost brother Toya.

As seen from their past, despite their supposed positive relationship, its been implied that Toya himself didn't initially have a good impression of his younger brother when he was born due to having figured out that his father was trying to make his new "replacement" in having more children.

Fuyumi Todoroki
Natsuo has a good relationship with his older sister Fuyumi, and as with Toya, they usually played together as kids. The pair still get along in their older years, with Fuyumi regularly teasing him in their visit to their mother, to his dismay.

However, unlike him, Fuyumi is able to look past their father's faults. He knows that she is trying so hard to fix the family that Endeavor broke, and that everyone can live together as a real and normal family. For this reason, for Fuyumi's sake, he is willing to begrudgingly participate in which his father is present, to respect the feelings of his sister, although this inevitably generates tension and Natsuo usually leaves to avoid more problems.

She also expressed concern for him after he was attacked by a villain, which he assured his frantic sister.

Shoto Todoroki
As a child, Natsuo could barely interact with with his younger brother because their father avoided any interaction between them since he had to train Shoto to make him stronger and surpass All Might. Once they grow up, they begin to interact more frequently, and in general their relationship is quite good, due to their shared resentment of their father.

During the Pro Hero Arc, he spent time with his sister and brother until their father returned home. Natsuo soon told off Endeavor and blaming him for his previous treatment, making him unable to know much of his brother. A surprised Shoto noted that he never saw his brother become as emotional as he did. He also seemed to sympathize with his Natsuo's point.

Months later, after a dinner at the Todoroki Abode, Shoto was concerned for him after he was attacked by a villain. With help from Izuku and Katsuki, Shoto secured his brother's safety and defeated Ending. After the attack, Natsuo thanked him and his friends for saving him.

Izuku Midoriya & Katsuki Bakugo
Natsuo meets Shoto's classmates when they were invited to have dinner with them at Todoroki Abode. During dinner there was hardly any interaction between them since Natsuo did not stay long in the place.

This changes after they, along with his brother, rescued him from Ending. Natsuo is grateful that they saved his life, especially to Katsuki who was the one who rescued him directly, asking him what his hero name is, although Katsuki doesn't tell him.