Tsumionji

Tsumionji is a femslash ship between Mikan Tsumiki and Hiyoko Saionji in the Danganronpa series.

Canon
''' WARNING: This section contains heavy spoilers for Danganronpa 2. '''

For most of the game, Hiyoko is abusive towards Mikan, bullying her and calling her names like "pig barf". Mikan never defends herself against the bullying, which in turn enrages Hiyoko and causes her to bully her more.

During the Twilight Murder Syndrome section of Chapter 2, it is revealed Mikan and Hiyoko used to be friends in Hope's Peak, alongside Ibuki Mioda and Mahiru Koizumi, although how their relantionship was at this point is not shown.

During Chapter 3 Mikan is reverted back to her Ultimate Despair personality and murders Hiyoko when she walks in on her arranging Ibuki's crime scene.

Fanon
Hiyoko's insults towards Mikan are usually aimed towards her body, such as calling her a slut for landing in unusual compromising positions. This may indicate Hiyoko having some sort of attraction towards her which she covers up with bullying.

Members of Ultimate Despair are said to take pleasure in their own despair, and this includes killing their loved ones. Therefore Mikan murdering Hiyoko can be interpreted as her expressing her love, specially considering her other victim was her close friend Ibuki.