Hiyoko's friendship with Mahiru blooms in Chapter 2, when Mahiru was the only one who offered to help Hiyoko wear her kimono properly and helps her take a shower. Since then, Hiyoko regularly referred to Mahiru as her 'big sis' or 'big sister Mahiru'. Hiyoko states that Mahiru is the only person who isn't weird and acts very casual.
Although Mahiru was initially reluctant to be dragged around by Hiyoko, it is evident that Hiyoko greatly looked up to the other as she was devastated by Mahiru's death, and later made her a memorial. Even after the events of Chapter 2, Hiyoko tries to take a shower and put her clothes back on by herself since Mahiru had taught her with so much patience, and was extremely frustrated to find out that she couldn't.
Later, it's revealed that Mahiru was one of Hiyoko's best friends during her time at Hope's Peak Academy as well, calling her "Big Sis" and stating she loves her. Despite their close friendship, Hiyoko involved Mahiru in her mischievous plot which included aphrodisiacs. Hiyoko later comforted Mahiru after Sato's death.
Upon awakening from their coma, Hiyoko is seen happily chatting with Mahiru and celebrating with her, Mikan, and Ibuki on the ship after the battle has ended.
In Danganronpa 3 relationship chart, Hiyoko tells Mahiru that she loves her, and Mahiru tells Hiyoko that she loves her too.
According to Kazutaka Kodaka, Hiyoko's school memories with Mahiru are very precious to her, explaining why she became close with Mahiru even after losing her memories in the Neo World Program.
In the non-canon "Danganronpa V3" bonus mode Ultimate Talent Development Plan, Hiyoko stated she feels lonely when Mahiru isn't around and that looking at her photos makes her feel better.
The ship came to existence as a result of Mahiru and Hiyoko's friendship, which developed at the start of Chapter 2. Mahiru being the only person whom Hiyoko liked and respected, as well as Mahiru defending Hiyoko when the others pointed out the latter started stinking also helped. The ship was even further fueled by the bonus scene in which the took took a shower together, which Hiyoko seemed to enjoy. Said scene even created the nickname for the ship, used as the title of this page.
In their fan works, fans tend to drop their "big sister/little sister" like relationship in favor of an actual sexual attraction to each other. Sometimes it's portrayed as a one-sided attraction on Hiyoko's side.
- Mahiru Koizumi/Hiyoko Saionji tag on AO3
- Hiyoko/Mahiru tag on FanFiction.Net
- Koizumi/Saionji tag on FanFiction.Net
- Hiyoko's relationship with Mahiru on Danganronpa Wiki
- Mahiru's relationship with Hiyoko on Danganronpa Wiki
- Both were the victims of back-to-back chapters in Danganronpa 2. Additionally, they almost died back-to-back, with only Peko being executed inbetween.
- Both grew with only one parent they could rely on, Mahiru with a mother and Hiyoko with a father.