The Flame Hero Endeavor is Shoto's father. Shoto's relationship with his father was on negative terms as Endeavor only saw Shoto as a tool for surpassing All Might. Shoto despised his father for seeing him as nothing more than a tool and resented him for his abuse against him, and his mother.
At a very young age, Endeavor began training Shoto, but the training was harsh and detestable. This was where Shoto began to dislike his father, telling his mother that he never wanted to become a Hero like him. Shoto's resentment of his father only further increased when he started to abuse his mother, as well as prevent him from interacting with his siblings. After Endeavor sent his mother to the hospital, Shoto snapped, and outright said to his father that he hated him, blaming him for everything that had happened.
Shoto's hatred towards his father shaped the majority of his early personality. He was shown to be incredibly rebellious towards him, and always had his father in his thoughts, wanting to deny him in any way that he could. Shoto's hatred was so deep that he refused to use his fire powers because he inherited them from his father, seeing it as his father's Quirk rather than his own. His refusal to use his fire powers came from his desire to deny his father the satisfaction of him surpassing All Might by relying on his fire Quirk, thus denying Endeavor achieving his goals. In his original Hero costume, he covered his left side with ice, as a sign of his rebellion towards his father.
Although Shoto resents his father for his lousy personality, and what he put him through in his childhood, Shoto is not blinded by his hatred towards him, and does respect his father for his intelligence, judgment, and insight. He chose to attend Endeavor's agency for work experience, as he believed that it would show him a lot about what it meant to be a Hero.
Despite this, Shoto has not forgiven his father for anything that he has done, however he no longer allows this to rule over him as a person.
Shoto also witnessed his father reacting in fury after he became the next No.1 Hero after All Might's retirement.
Enji later expressed pride in his son after witnessing his growth during the Provisional Hero License Exam Remedial Course, though Shoto gave him the cold shoulder. Undeterred by this, Enji simply told him he would try to be a better hero and man that Shoto could be proud of as the latter simply shrugged him off but smiled at his father's change.
Shoto continued to watch his father the Nomu and even cheered him on before he saw him emerged victorious. Relieved to see this, Shoto fell to his knees and was happy for him.
Shoto and his father met again after his battle with High-End, Shoto took time out of school to come home. He noted on Enji's new scar, in a rather tense tone, as if to imply that Enji now knows that scars are permanent. After Natsuo confronted Enji and left, Shoto spoke up and called him a great hero after his success. Shoto surprised Enji by voicing while he still hasn't forgiven him yet, he looks forward to see the changes he will make in being a father.
Unlike his father, Shoto's mother was kind, caring, and loving towards Shoto, and in return Shoto loved her deeply.
Shoto hated his father for constantly abusing his mother. Endeavor began to harshly train Shoto when he was only 5. Shoto's mother often intervened, trying to stop Endeavor's mad training methods, but failed. Shoto's mother supported Shoto in these dark times, telling him that it was okay for him to become a hero.
Shoto's mother soon went insane from Endeavor's abuse, and started to hate him as well as Shoto's left side. One day, she finally snapped, and threw boiling water over Shoto's face, giving him a burn over his left eye. Shoto bore no ill will against his mother for this incident, and displayed no hatred for her, and instead blamed his father. Shoto asked where his mother was, and became shocked that she was admitted to the hospital. During his rebellion towards his father, he referred to his ice powers as his mother's Quirk, indicating that he was trying to become the number one Hero using only his ice powers for his mother's sake as well as his own.
Even after everything that happened in the past, Shoto still loves his mother. Shoto meets with his mother after the Sports Festival and she reconciles with her son. Thus, their relationship returned to what it was in the past: a loving relationship between mother and son. Thanks to this, Shoto has come to terms with using his fire power. He also frequently visits her on his days off.
As a child, Shoto always watched his older brothers and sister played, often wanting to join them as they had fun but his father prevented any interactions between them since he had to train.
However, he gets along with his sister Fuyumi Todoroki and they appear close enough for him to tell her that he was visiting their mother.
During the Pro Hero Arc, he spent time with his sister and brother Natsuo Todoroki before he watched as the latter told off their father for his previous treatment to him and his siblings. Shoto noted that he never saw his brother become as emotional as he did. He also seemed to sympathize with his sister's desire of being a family.
Shoto initially saw Izuku as his arch-rival due to him having connections with All Might (a person whom his father has a rivalry with). Shoto's interactions with Izuku were of a serious nature, but not to the point of antagonistic; after Stain's defeat, Shoto becomes friendlier with Izuku.
After Izuku made him use his fire power during the Human Cavalry Battle, Shoto started to take Izuku more seriously, going so far as telling Izuku his back story and declaring to him that he would surpass him using only his ice powers and in the process disown his father. Izuku has a lot of respect for Shoto, admitting that he is far stronger than him. Izuku began to show sympathy for Shoto after seeing his sadness and resolved to help him, rather than focusing on winning. During the final clash in their battle, Shoto thanked Izuku for helping him see that he's not his father. After his fight with Izuku, Shoto begun to acknowledge Izuku's prowess, noting that he is a person compelled to help people no matter what.
Shoto has gained respect towards Izuku for his strength, and how he acts to help people, regardless of the risk and cost to himself. He is willing to go along with any plans Izuku formulates, showing that he trusts his judgement. In the Vs. Hero Killer Arc, when Izuku sends out a message for emergency back-up, Shoto is the first and only to arrive, telling his father that his friend needed his help, showing that he views Izuku as a friend now. Shoto was even amazed by Izuku gaining more control of his quirk.
After the Final Exams, Izuku is seen worrying about Shoto's feelings, especially towards his father. During the Hideout Raid Arc, Shoto is seen reciprocating these with his own reassurances and support, such as the time he told Izuku that "It's okay for heroes to cry" and taking Izuku's feelings into consideration, asking if he wanted to join, when he and Eijiro were planning to save Katsuki.
Shoto and Katsuki have interacted a few times such as in the Obstacle Race during the Sports Festival where they had a brief scuffle for first place, and before they faced each other in the final match of the Sports Festival's tournament event. Shoto appears to not be fond of Katsuki's aggressive personality and doesn't seem to see Katsuki as a rival. On the other hand, Katsuki was shown to be envious of Shoto's strength after seeing him in the Battle Trial. However, Katsuki dislikes Shoto's cold attitude. Unlike Izuku, Katsuki doesn't care about Shoto's problems or feelings. It seems that by the end of the first semester, Shoto and Katsuki appear to be neutral with each other, enough for them to have limited social interactions with each other without hostility as shown when they joked to each other about Kota reminding them of people they knew.
During the Villain attack during the trip to the forest lodge, Shoto and Katsuki were paired together for the test of courage, and were therefore separated from everyone else when the League of villains attacked. Shoto tried to keep Katsuki from engaging in battle, reminding him of the various dangers of using his Quirk near the gas, and to stop him from being taken captive, though Katsuki showed anger at Shoto for doing so. Shoto also attempted to save Katsuki from being kidnapped by the League of Villains and agreed to help rescue him from his captors. Since his rescue, Shoto has shown to be more friendly towards Katsuki, attempting to start casual conversations with him, shown when they both had to go to extra lessons to get their provisional licenses. However, Katsuki still shows annoyance towards Shoto for doing this. During the Remedial Course Arc, when Katsuki proposed to be more violent with the delinquent children, Shoto proposes that there are better ways to deal with the situation. This made Katsuki think back about Shoto's past and allowed the latter to proceed with his approach, which showed that he is at least more sensitive to Shoto's past.
Shoto appears to be on good terms with Tenya, respecting Tenya for his incredible speed after their battle in the U.A. Sports Festival; Tenya appears to also respect Shoto for having the behavior of a true Hero. They are first seen interacting with each other when Shoto chose Tenya to join his team in the Cavalry Battle. After the Sports Festival, Shoto becomes increasingly worried about Tenya's mental health when he heard about Tensei Iida's injuries from Stain. Shoto understood Tenya's hatred for Stain fully well as he too knows the feeling of hating someone and thus could relate to him, deciding to help Tenya overcome his bitterness such as encouraging Tenya to become the man he wants to be.
After the battle against the Hero Killer, they are often seen together along with Izuku Midoriya, meaning they view each other as friends.
Although their interactions have been minimal, Momo and Shoto are generally friendly towards one another. Shoto has many similarities to Momo; they both are students who got into U.A. through recommendations, hail from famous Hero families, and have powerful Quirks.
Shoto clearly respects Momo's intelligence and abilities. He chose her for the Human Cavalry Battle in the U.A. Sports Festival, and voted for her during the class rep election. Shoto even told Momo he believed that she excelled at the role of being a leader and gave a vote for her in being class representative.
During the practical portion of the Final Exams, Shoto is partnered with Momo against Aizawa. He appeared to be oblivious that Momo's low self-esteem bridged from comparing herself to him. When his plan backfired, he apologized for not asking if Momo had an idea as well. When Momo expressed her doubts out loud, Shoto comforted her by telling her that he respected her, and to have faith in her own capabilities. After being captured by their teacher, Shoto encouraged Momo to believe in herself as a leader. They worked together to outwit Aizawa and managed to pass their finals.
Shoto was disgusted with Stain's ideals of killing "fake" Pro Heroes in order to achieve justice, going so far as to tell Tenya to pay no heed to him. Stain appears to respect Shoto's strength and selfless attitude to save his friends.
Shoto was conceived and trained by his father for the sole purpose of surpassing All Might, however while Shoto does express this desire, it does not appear to be as heavy as Endeavor's; rather, Shoto respects and admires All Might. When he was younger, he was inspired by All Might's words of an individual's Quirk being their own despite lineage and wants to be a hero just like him.
Shoto once saved All Might from being killed by Nomu and eventually figured out that Izuku has a connection to the hero. In return, All Might is amazed by Shoto's skills and seems to have noticed the boy's troubles and does his best to offer support to Shoto without prying too deeply into his personal issues.