Both were classmates in Class 77-B, and participants of the Killing School Trip.
Similarly to Mikan Tsumiki, Hiyoko finds Kazuichi an easy victim to pick on, and is quick to begin bullying him. She often patronizes him with insults, referring to him most of all as 'useless,' different from the insults she often calls Mikan.
Hiyoko seems to enjoy bullying him, often taking the chance to do so, and sometimes choosing to do so without any prompting whatsoever. A few times, Hiyoko even states that she wishes to see Kazuichi hurt and threatened him herself.
During the first chapter of the game, after the execution of the first culprit, Kazuichi expresses great sadness over the loss of a classmate. However, after a few snide comments by Hiyoko, Kazuichi is forced to push away his shock.
During the second chapter of the game, Kazuichi and Hajime Hinata witness Hiyoko fleeing from the crime scene of one of her dead friends. Later, during the trial, Kazuichi and Hajime's eye-witness account allow Hiyoko to escape framing by the true culprit of the second case. Kazuichi points out that as Hiyoko was running away, she had no blood on her or her clothes. This allowed Hiyoko to become partially cleared of her initial suspicion.
During the third chapter of the game, neither Soda nor Hiyoko are affected by the Despair Disease motive, and move to the Motel with the rest of the uncontaminated people. Despite the time Hiyoko has shared with the majority of her class, she does not lessen her bullying of Kazuichi.
The two do not interact any more after the third chapter, however, because Hiyoko becomes one of the two victims of the third blackened.
During Hiyoko's free time events with Hajime, she complains that Kazuichi is 'a pathetic, annoying virgin' who is especially annoying when he's around his crush, Sonia Nevermind. Some may view this as Hiyoko being jealous, however, it can also be interpreted that Hiyoko simply takes issue in how idiotic and reliant Soda becomes on Sonia when she is present.
Similarly to Tsumionji, Soudaionji is not a vastly popular ship due to Hiyoko's treatment of Kazuichi. However, there are still those that choose to ship it.
Due to Hiyoko's complains about Kazuichi's behavior towards Sonia in specific, some view her bullying in that respect as a form of jealousy. However, the hints towards that are very unclear.
- Hiymahsouda refers to the ship between the two and Mahiru Koizumi.