Hiyoko Saionji is a character featured in Danganronpa 2: Goodbye Despair. Even though she is more of an anti-hero, she also has some antagonistic acts(especially as a Remnant of Despair while brainwashed).
She is really only nice to three people. She sees the main character, Hajime Hinata, as a big brother. When she first appears in the game she is killing ants in a childish way. She was part of the reason why Peko Pekoyama, Mahiru Koizumi, Sato, and Ibuki Mioda were killed.
Later on, Mikan Tsumiki turns into a psychopathic killer who killed Hiyoko after realizing that Hiyoko witnessed her killing Ibuki Mioda.
She also cries to manipulate people into things because everyone was being mean. For the scale of being mean, she likes to bully people even after their death and likes killing insects.
She is also one of the ones that force people of all ages to kill themselves and rips up dead bodies. In the end of the game she is in a coma where she is still alive with the other people that were thought to be dead, but not 'living' either.
Before the events of the game and before she lost her memories Hiyoko was a part of the Remnants of Despair which were a group that would brainwash people, kill them, even kill their own families, steal stuff.