Saika Gasai

Saika Gasai (我妻 西果, Gasai Saika) is the adoptive mother of Yuno Gasai and the wife of Ushio Gasai. A minor character with major relevance, she serves as the direct cause of Yuno's descent into insanity.

Personality
Although she is only briefly seen, it is known that Saika was a very strict and abusive mother towards Yuno. This comes from the fact that she feels herself lonely and neglected by her busy husband, thus taking her frustrations into Yuno punishing harshly locking her in a cage and feeding her little to no food, calling her a bad daughter. In the anime she is seen to both psychologically torture Yuno saying that she disobeyed her orders to make her suffer and for her to suddenly burst into tears and ask for forgiveness that she's a bad mother, showing she's in fact mentally unstable as a result of the hardships of her family life.

She was then drugged by Yuno and locked up with her husband Ushio Gasai in the same cage she used to lock up Yuno, who believed that her mother would change if she suffered the same pain she had. At this moment Saika becomes much more hostile and maniacal, calling Yuno a devil, telling her she curses the day she was born, with great anger and little self-consideration for what she made her daughter endure.

During the end of the series she proves nonetheless to be a loving mother and wife, who would put her own life in the line to save her family.

Behind the Scenes
It is known that years before the series timeline, Saika and Ushio decided to adopt Yuno as they were unable to have children for unknown reasons. Although initially a loving mother according to Yuno, this changed over time as she felt neglected by her husband who spent less and less time with his family as a result of his busy schedule. Saika would then take her frustrations into Yuno with a strict curfew and locking her up in a cage and feeding her little to no food at all as punishment. Given her mental instability it is also possible (albeit unconfirmed) that she may have locked Yuno even if she was an obeying daughter, as she would call her a bad daughter and measure the number of times she "talked back" as part of her regulations and punishment. Yuno endured this abuse for years knowing that her mother was suffering, but eventually has enough and locks her and his father in the same cage, with the same treatment, thinking the same pain would make them loving parents again. Far from this however Saika would call Yuno a demon seed, before dying from starvation and/or dehydration. Her constant abuses and ever growing hostility towards Yuno before and after her imprisonment serve as one of the major causes if not the most important for Yuno's insanity.

Even dead she serves as a major importance in the series in future events related to Yuno, namely that Aru Akise would investigate Yuno and Yukiteru Amano becomes doubtful of her to the point of believing her to be a psychopath.

The Final Battle
She's seen 2 years before the series timeline abusing Yuno. She however would be called and taken by Keigo Kurusu upon receiving reports. When Kurusu goes to the Sakurami Middle School in where the final battle of the Survival Game is taking place, Saika rushes out of the car to look for her daughter despite Kurusu's warnings. She finds the first Yuno trying to kill her third self but is protect by Ushio who is stabbed instead, and pleads with Yuno to take her life instead of her family's.

Finally after the Survival Game ends, the incident that nearly destroys their family, seems to have made them a much happier and united family as Yuno is no longer abused and leaves a fairly normal life. Because of this it can be hinted that she becomes either a devoted mother, and/or she works along her husband as she is herself an owner of the Gasai Bank, but nonetheless is a better and happier woman.