The enigmatic love interest of main character, Renton Thurston, Eureka takes the form of an adolescent young woman.
Eureka is an alien species known as a Coralian that is a part of the planet. The Coralians wanted to know more about the human species since they did not want to frighten them away, so they created Eureka as a "blank copy" that was Coralian on the inside and human on the outside. Her role was to gather information on humans and see how Coralians could interact with the human species without causing them to leave the planet like they did 10,000 years ago before the start of the series. Since the military found her as well as the first LFO the Nirvash, they did research on her in order to comprehend who she was and what she was. The main researcher, Adrock Thurston, oversaw everything to do with Eureka and was with her when the Summer of Love incident occurred where he mysteriously vanished.
After that, Eureka was put into the S.O.F. in the United Federations with Holland Novak, Ray and Charles Beams, and Talho Yuki under the command of Dewey Novak. However, during a raid on the city of Cuidades del Cielo, she killed what she thought was a group of terrorists when in reality is was a bunch of people protecting children. Her guilt overtook her as she adopted the children and named them Maurice, Maeter, and Linck. Discussing it with Holland, some members of the S.O.F. took her, the Nirvash, and the ship the Gekko and fled to become an anti-government group. During this time, Holland believed that he was supposed to be the chosen one that was meant to be Eureka's partner but this proved not to be the case later on.
Eureka was created by Tomoki Kyoda with the purpose of being the main female lead in a mecha anime about finding oneself and a personal journey. Her name comes from the phrase "Eureka", a Greek phrase referring to an idea being discovered in a bout in inspiration. It should be noted that while they look the same, they are pronounced differently. (The standard way of pronouncing it would be 'you-reek-a' but with her name, it is said 'el-wreck-ah').
In the beginning of the series, Eureka acts extremely distant and odd for a standard teenager due to the fact that she has had very little interaction with humans outside the military. She seems to act as if she has no emotions or care towards anything, nonchalantly going along with anything anyone suggests. However, things change immediately when she meets Renton Thurston. Eureka sees something different in him as she finds that he is also able to pilot the Nirvash alongside her. As time goes on, she begins to act less like some socially awkward girl and more like an actual person as parts of her human side begin to emerge.
For a time, Eureka felt distant and lonely on the Gekko due in part to Renton bottling up his own emotions and feelings and acting cold to Eureka due to his own problems. He is able to save her from the Coral and leaves as he blames himself for the problem. However, this causes Eureka to change from an uncaring girl to someone more sympathetic. When she discovers Renton left, she bursts into tears and blames herself for his exodus. They are able to meet once more as they save one another from a military attack. Renton wonders why all this is going on as Eureka claims that she 'chose' him. Soon after, Talho reveals Eureka's past as a Coralian, something that Renton still accepts.
Despite being together with Renton, Eureka feels remorse for dragging him into a war and staining his hands with blood. He does his best to console her but there are still human things that she lacks the knowledge of. Renton continues to help and support Eureka in her times of need when she feels helpless or that Renton will be disgusted by her, claiming that he loves her for who she is. The two travel to Earth with the children when Eureka realizes that she also loves Renton, sprouting wings and spotches of green orbs on her body. Eureka is shown to sacrifice herself if it means that the people she love will be safe, doing a complete 180 in a sense.
Eureka Seven: Pocketful of Rainbows
The humans are fighting against The Image and Eureka and Renton are fighting to make their dream come true.