Eureka is the female protagonist of the manga and anime series Eureka Seven and Eureka Seven:AO. She is the love interest of the main male protagonist, Renton Thurston.
Eureka is a pilot of the Nirvash typeZERO. She treats Nirvash not like a mere machine, but as a living being, with consciousness and feelings. She meets Renton when Nirvash crashes on his house and he falls madly in love with her from the moment he sees her.
Eureka can seem distant and awkward to other people - she has trouble communicating with other humans and understanding their feelings. She tends to appear randomly next to people, startling them at times. Initially in the anime, she suffers from frequent headaches; these stop later on, seemingly due to the presence of Renton. She is the adoptive mother of Maurice, Maeter, and Linck.
It is later revealed that she is actually a Coralian in human form, sent by the Scub Coral to understand and communicate with humans. To learn about humans, she was born with no memories, emotions, and common abilities, so she can have more experiences with humans and develop her own human-like personality.
Prior to meeting Renton, Eureka had little contact with humans beyond the military. This and her being born with literally no memories or personality, she was blunt, aloof, and unemotional. She initially lacked empathy for others and often misunderstood the actions of everyone around her, due to her lack of knowledge on human nature. The only people she trusted were her adopted children and Holland, but after meeting Renton, she grew to accept the rest of the Gekko crew and developed a good relationship with all of them. Through her relationship with Renton, she started to learn several new emotions from love, hate, jealousy, sadness, and so on. She credits Renton for teaching her the true meaning of life and how to love others. She used to be emotionally dependent on Holland, often letting him make all of her decisions and being unable to do anything that went against his orders, even when she knew he was hurting Renton, whom she was starting to fall in love with. Eventually, she learns a bit of independence and she doesn't need to rely on Holland for everything, even though she becomes a little dependent on Renton. Despite her lack of knowledge on raising children and feeling it is her responsibility to raise them as atonement for killing their families, Eureka really does love the children and treats them as if they were her own, even though she tends to spoil them. However, for the rest of her life, she is plagued with guilt for her past sins and shame for her own identity as a Coralian. Overtime, she goes from a socially awkward and emotionless girl to a brave, compassionate, and kind young woman who is willing to do anything to protect those dear to her and the planet she comes to love. After becoming a mother, Eureka develops an overprotective and loving attitude towards her son and daughter.
Eureka first met Renton when she crashed Nirvash into the Thurston Garage and Renton fell in love with her at first sight. However, due to her lack of knowledge on human emotions and nature, she didn't reciprocate his feelings at first. Once he joins Gekkostate, Eureka develops a friendship with him, finding herself interested in his amusing and good-hearted personality. Their relationship is nearly broken when they have a severe falling out over Eureka rejecting him out of jealousy towards him for supposedly taking her place as Nirvash's pilot and blaming him for causing the change, which she deemed as painful, in her. Finding it hopeless to get through to Eureka, Renton left and Eureka didn't find out until several days later, which left her guilt-ridden and sent her into a deep depression, to the point where all she did was cry and mourn. She later left the ship to find him after concluding she wanted to be with him forever and upon reuniting, she told him he was meant to be with her and he reciprocated her feelings. They began a relationship and Eureka quickly fell in love with Renton. Their relationship becomes serious right away, to the point where they contemplated marriage and having a family despite being 14, and take on roles as parents to the kids. They also take their committed relationship serious by risking their lives to travel past the Great Wall and enter into the Promised Land to find a way to unite the humans and Scub Coral. Renton is the one who suggests his and Eureka's relationship can serve as an example of the two species living together peacefully. However, due to the efforts of Dewey and the military, Eureka almost sacrificed herself to save the planet from the Limit of Questions until Renton, risking his own life, came to find her and they vowed to stay together no matter what, leading them to share their first and second kiss, and their love is able to activate the Second Summer of Love and save the planet. In the process, their names and a heart were carved into the moon. A year later, Eureka married Renton and they became official adoptive parents to the kids. A few years pass before Eureka has two children with Renton, a son named Ao and a daughter named Amber.
Despite still being truly in love with Renton, their marriage was a sad one, starting with the revelation that human-Coralian hybrid children will die in their trapar-invested world. Their daughter, Amber, dies as an infant and they were forced to give their son, Ao, up by having him raised in an alternate universe. In a series of events, Eureka was separated from Renton before giving birth to Ao and left to raise him as she waited for Renton to find them, and she never gave up on reuniting with him again. They are finally reunited at the end of Eureka Seven: AO but were forced to leave their son as he refused to leave the world he grew up in. Despite everything they've been through, Eureka insists she doesn't regret falling in love with Renton and having children with him.
- Eureka's true age is unknown and she retained the appearance of a teenage girl from the time of her discovery until the end of the series. Some of the Gekko crew guessed her age to between 14 and 16. It was later revealed she was found the same year Renton was born, so she was given the same age as him when she married him. After marrying Renton and having children, she noticeably aged.
- In the film, Eureka Seven: Pocketful of Rainbows, Eureka's last name was Zita. In the series, she didn't have a last name until she married Renton and became known as Eureka Thurston.
- Her wedding anniversary is April 2, 12006, and the paperwork listed her age at the time of her marriage as 16.
- In episode 39, Eureka is shown to be skilled in soccer, despite having never played before.
- In Eureka Seven:AO, her attire is similar to the one her sister-in-law, Diane, wears.
- Eureka stated in both E7 and AO that what she loved most in the world were Renton and their children (2 biological and 3 adopted). She also stated that her dream was to be a normal person and have a family with Renton.
- Even though Eureka has 5 children with Renton, it appears Ao is her favorite, likely due to him resembling Renton and being the only on of her two biological children to survive, and she affectionately refers to him as "my Ao".
- Eureka is named after a famous exclamation attributed to the Greek sage Archimedes.
- Even though Eureka loves her adopted children dearly, she never mentions them in AO.
- In episode 19, Eureka attempted suicide by almost allowing the Scub Coral to absorb her, due to her depression of being estranged from Nirvash and her belief she is hated as a result of Renton changing her. She went as far as to compare herself to a book with blank pages to emphasize her belief she had no purpose in life. However, the thought of dying scared her and she wanted Renton to save her, which is what he did.