Himiko's relationship with her parents is very poor. They were disgusted by their daughter's blood interest since she was a child, and demanded her to stop such behavior and simply be "normal". They did everything they could to suppress her "abnormal" tendencies, putting immense effort to repress it, successfully hiding this side of herself for years.
However, their efforts were in vain and only served to ruin their relationship with their daughter. Toga ended up attacking and drinking the blood of one of her classmates. This incident had presumably put her parents and other family members in bad standing with the public at large. Due to the media identifying her and later interviewed them regarding their daughter's behavior, her parents attempted to deflect the negative public opinion on them by publicly apologizing for what happened and publicly cut ties with their daughter. They then announced that they have always considered her a lost cause, and ended up branding her as a "creepy demon child" and tried to deflect any blame on their parenting by claiming that she was born bad.
Upon their first meeting, Tomura calls Himiko a brat and freak due to her excited attitude as well as for her abnormal fanaticism over Stain. Giran comments that Himiko can be useful. However, unimpressed and in a bad mood, Tomura tries to attack her and Dabi but the three are stopped by Kurogiri. After Tomura's departure from the meeting, Himiko confesses that she thought she was going to die.
Despite the chaotic first meeting, Tomura accepts Himiko and Dabi into the League of Villains. When he tells Kurogiri that it doesn't matter if the Vanguard Action Squad succeeds or fails and Kurogiri responds with describing the group as "disposable pawns" Tomura interrupts, revealing that he sees Himiko and the other as reliable comrades with useful strengths.
When Overhaul requests Himiko and Twice to take part in his plan, Tomura responds that they are his "cornerstones". Upon hearing she has to work for Overhaul, Himiko reveals that the League makes her feel good and that she wants to live however she feels like in the world. She playfully pulls a knife on Tomura while asking why does she have to do things she doesn't want to. Tomura pulls off his hand revealing a calm and decided face. He tells her and Twice that they will help Overhaul for him, themselves and the rest of the League and that he believes in them. This talk results in Himiko and Twice creating the havoc that Overhaul tried to avoid.
Both Himiko and Dabi are influenced by Stain's ideology however, for different reasons. Himiko believes the world is difficult to live and she wants to make it easier. Her idea of making life easier is being able to do as she pleases (killing).  Dabi, however, wants to make Stain's dream of exposing the faults of hero society a reality. 
Though the two are shown to be able to get along, it should be noted that they often exchange verbal jabs. Himiko tends to make fun of Dabi every time his plans or actions do not go as he would like. Himiko comments that Dabi must be bad at judging people when Twice notes Dabi only incinerates possible recruits. She teases that Dabi won't return to fight Gigantomachia because his flames had no effect. Meanwhile, Dabi tends to call her an assortment of names such as "wackjob" and "nutjob".  
Himiko and Twice are shown to get along well. During their internship in the Yakuza organization, Himiko shows a great understanding of Twice's feelings about them working together with the Yakuza. Himiko repairs Twice's mask, after his confrontation with Sir Nighteye and reminds him that if they don't cooperate they will never get the chance to get revenge for Magne.
The two decide to 'do things their way' which results in them sabotaging the Yakuza's plans by angering Mimic to losing his concentration leading to his capture as well as leading Ryuko's squad to aid in the battle against Overhaul.
Twice develops feelings for her because of the sympathy she showed him during the their time as Yakuza. He calls her his soulmate after she repairs his mask. Twice later questions if he can have a kiss as they look for an exit out from the underground paths. Himiko doesn't respond to his affections. However, she allows him to dress her in the fake jewelry stolen from the Creature Rejection Clan.
When Himiko's life was threatened by the Skeptic puppets, the desire for Twice to save her life was so strong that he started overcoming his trauma and creating an army of Doubles of himself.  and later he creates a Double of her to give her a blood transfusion. When the battle begins to worsen, he takes her away from the danger. One week after the Revival Celebration, after the League and the Army decided to ally forming the Paranormal Liberation Front, Twice mourns over Himiko Toga’s Double death, which annoys the real Himiko, who is alive.
Mustard considers Himiko to be impractical, as he criticized her for caring more about whether or not her support gear was cute than for the equipment's functionality.
Himiko develops feelings for Izuku after seeing him badly injured while being carried by Mezo Shoji during Vanguard Action Squad's attack. Izuku's appearance matched her type of "tattered guys reeking of blood". The League disperses after the training camp attack.
Later, Himiko enters the Provisional Hero License Exam impersonating Camie Utsushimi. She confronts Izuku during round one asking why he wants to be a hero. She questions if it is for honor, pride or for someone else. When more students show up Himiko disappears and returns as Ochaco Uraraka. She calls out to Izuku but falls causing him to save her. Hiding among the rubble, Izuku confronts Himiko. He tells her that he knows she's not the real Ochaco because Ochaco wouldn't act without a plan nor forget to use her Quirk. He admits that he saved her because he didn't want her to get hurt despite knowing she wasn't Ochaco. Himiko responds with "I see…so that was your reason. You've got to let me get to know you better."
During her internship with Shie Hassaikai, Himiko becomes elated that Izuku remembered her. She admits that she's glad to see him and couldn't be any happier.. Later, while leaving with Twice, she confesses that she still wants to see Izuku (looking all beaten up). After Overhaul's arrest she calls Mr. Compress obsessing over Izuku, commenting that she couldn't take her eyes off him before relaying information to Mr. Compress. 
Upon meeting Ochaco and Tsuyu Asui she proclaims that both are cute and lovely. Himiko says that Ochaco has the same scent as her (love) after being subdued by Ochaco. Himiko continues with how she knows Ochaco wants to be just like the one she's in love with and how it's only natural. When more students arrive Himiko quickly gets out of Ochaco's submission hold and goes to join up with Dabi and Twice. Twice questions why she's in such 'high spirits'. Himiko confesses that she made a friend (Ochaco) and met a boy (Izuku).
During the Provisional Hero License Exam, after her second confrontation with Izuku, Himiko, disguised as Camie, tells Ochaco that Izuku really does trust her.
Himiko admits to loving Ochaco and wanting to be her. 
As with Ochaco, Himiko shows interest in Tsuyu as soon as she meets her, and tries to become her friend in her twisted way, that is, attacking her with the knife to make her bleed. Tsuyu greatly dislikes Himiko for attacking and injuring her and Ochaco, and considers Himiko nothing more that a crazy girl. Despite Himiko's attempts to "befriend" her, Tsuyu reacts with displeasure, especially when Himiko called her by her first name, since that's something only her friends are allowed to do.
Although she did not know him personally, Himiko harbors an unhealthy obsession towards Stain. This is the reason why she decided to join the League of Villains. Himiko declares to Tomura that she wishes to both become Stain as well as to kill him. During her fight with Ochaco, Himiko notices that she is in love with someone, so she begins talking about people who are in love strive to be like the ones they love and what kind of people she likes. Himiko declares that she loves "tattered guys reeking of blood" while thinking of Stain.
Himiko hates Overhaul due to the fact that he killed Magne and destroyed Mr. Compress's arm in his first meeting with the League of Villains. When Tomura tells her and Twice that they will work for Overhaul she playfully pulls a knife on him and asks why should she do things that she doesn't want to.
At the Shie Hassaikai Compound she admits to not liking any of the Yakuzas and refuses to tell them anything regard her Quirk. However, Shin Nemoto is able to get her and Twice to reveal their Quirks.
During the raid on the Shie Hassaikai Compound Overhaul orders her and Twice to hold off the enemy to cover his escape. They reluctantly obey. Himiko tries to take delight in her new duties while mocking the Yakuzas when she attacks Lock Rock while saying she's a "protected treasure from a golden era, Toga the Yakuza." However, Himiko and Twice decide to do things their way resulting in sabotaging Overhaul's plans. She confesses, while she and Twice make an exit, that she wants to see Overhaul suffer which leads Twice to come up with the plan to lead Ryuko's squad to Izuku's and Overhaul's battle.
After his arrest, Himiko relays information on where Overhaul will be taken to Mr. Compress so the rest of League can intercept the transport vehicle carrying Overhaul. The interception in successful and result in Tomura and Mr. Compress taking both their revenge and the Quirk-destroying bullets.
They first meet when Himiko and the League go to Deika City. When confronting Himiko, Chitose admits she's intrigued by Himiko Toga's backstory. She is downright obsessive with the idea of interviewing her, even in the middle of a deadly battle between the two; and she's quite amazed to watch her get more and more deranged as their fight goes on.
Himiko was annoyed by Chitose wanting to know how and why a nice girl like Himiko became a serial killer, and says she has already interviewed her parents and her former high school friends. Himiko declined Chitose's desire to use her influence to turn Himiko into a martyr that was shunned and repressed by the hero society, which led to her becoming a villain, furthering the agenda of the Meta Liberation Army.
She nearly kills Himiko by overwhelming her with numbers and hitting her with her Quirk repeatedly, but is caught off guard when Himiko transforms into Ochaco Uraraka and uses Zero Gravity to levitate her into the air and drop her from a lethal height. Even so, all she can think about is how amazing the article about Himiko she'll write is going to be, smiling in pure euphoria while falling.