Mana Ouma is the yandere to her younger brother, Shuu from Guilty Crown.
For the first part of plot, Mana is seen to be a sweet, gentle and caring young girl who loves her brother a lot. She is also seen to be kind and warm, as she rescued Gai without thinking of consequences and named him "Triton" as she saw him struggling to speak.
However, it is later revealed that Mana was evil and sadistic. Her such behavior was a result of the virus residing within her, which made her weak and strong at the same time. Mana's mind had turned completely different, and she ended up killing her mother at the age of seven and tried to kill Gai at twelve.
While in Inori, she's seen to be manipulative, entangling Shu to fall in love with her very easily. She's also inhumanly cold, as she kills people with a sadistic, maniacal smile spread over her face.
After her reincarnation, these negative characteristics had grown stronger and she was dominantly ruled by them, erasing any good quality she ever had.