Yui Ikari (碇 ユイ Ikari Yui?) is a fictional character from the Neon Genesis Evangelion franchise. She is Gendo Ikari's wife and Shinji Ikari's mother. Her soul is bonded to the Evangelion Unit 01, and it is implied on numerous occasions in the original anime that there is a connection between her and Rei Ayanami.
In episode 21, during Seele's interrogation of Nerv's Deputy Commander Kozo Fuyutsuki, he recalls his memories of Yui. She was a student of genetic engineering at Kyoto University, where she met Fuyutsuki, who took an interest in her research and would later become a mentor and trusted friend, as well as her future husband Gendo. Afterwards, she joined the Artificial Evolution Laboratory (which would become Gehirn) in Hakone, Japan, and played an integral role in Project E.
Despite Fuyutsuki's misgivings, Yui served as the test subject for the Contact Experiment of Unit 01 in 2004, saying that "everything happens as it must." She invited her son Shinji to Gehirn to watch what would be her final moments: Yui "disappeared" during testing — leaving no body behind — and was proclaimed dead that same year after efforts to recover her failed. Although Gehirn considered it an accident, it is arguably clear that what happened was Yui's intention. Living on as the soul of the Eva, she continues to ensure a "bright future" for Shinji and the rest of humanity, seeing to it personally that Seele's scenario is never entirely fulfilled.
Yui appears briefly in episode 16 as a pair of glowing arms which reach out to Shinji as he slips into unconsciousness in Unit 01's entry plug; the Eva tears its way out of Leliel's shadow immediately thereafter. In episode 20, while Shinji is absorbed in Unit 01, we see a static screen of a drawing of Yui breastfeeding Shinji. Yui and Gendo talk, and Yui sees and talks briefly with Shinji then urges him to leave the Eva and return to the world.
Yui's background is not covered in the series, but according to information provided in the Neon Genesis Evangelion 2 PS2 game, she is the daughter of an influential member of Seele, which explains why she had the backing of this powerful organization. However, the canonocity of this has never been officially stated.
Impact on other characters
Despite her limited appearances, Yui is one of the most important characters in the series. Gendo's love for her is his entire motivation for going through with Seele's Instrumentality Project. He misses her so longingly that he goes through with Seele's plan in the hopes that, after Third Impact, he would be able to be one with Yui again.
Shinji is equally affected by his mother's absence, as that and Gendo's subsequent abandonment of him is a massive trauma that he has carried his entire life and is what has led to his avoidant, non-confrontational nature. At the same time, her maternal protectiveness of Shinji as the soul of Unit 01 is solely responsible for the Eva being the formidable fighter it is.
Although Fuyutsuki was never more than a mentor and friend to Yui, it is strongly implied that he was in love with her to some degree, as he was shocked to learn that she was marrying Gendo (whom he disliked at the time); also, in The End of Evangelion Lilith/Rei takes the form of Yui to achieve Fuyutsuki's Instrumentality.
Rebuild of Evangelion
Yui makes a brief audio-only appearance in the first Rebuild of Evangelion film, as a voiceover speaking with Gendo during a dream/memory Shinji has after his first Angel battle. Yui makes a cameo appearance in the 2nd Rebuild film. Rei is eating with Gendo and asks him if she could set up a dinner party where together they could eat with Shinji and his friends. Gendo is cold to the idea at first but then, in his mind's eye, Rei forms into Yui who asks Gendo to look after their son. After seeing this, Gendo agrees to the idea.
Neon Genesis Evangelion
In Yoshiyuki Sadamoto's manga adaptation, Yui's role is identical to that in the anime, but is expanded slightly. In the aftermath of the fight with the Angel Zeruel, she appears inside Unit 01 and tried to convince Shinji to remain with her. Initially, it seems to be working well: Shinji refuses to come out when attempts are made to retrieve him, forcing the contents of the Entry Plug to spill out. However, Rei mentally calls out to Shinji, and Yui's face peels away to reveal that it is Unit 01 itself who has disguised itself as Yui to try to trap Shinji inside itself. Upon seeing Unit 01's true self, a horrified Shinji struggles with the Eva, crying out to Yui. A female form appears behind the disfigured body of Unit 01, and appears to merge with Shinji. Later, Shinji sees a scene from the past: His parents were under a tree, and Yui was breastfeeding an infant Shinji. After a short conversation with Gendo, Yui then notices her grown son and asks him to join her. Surprisingly, Shinji is unable to move, despite his desire to get close to his mother. Yui then convinces Shinji to leave the confines of Unit 01 and go back to the world.
While the manga is in black and white, the cover of volume 8 depicts Yui with reddish-blonde hair and light green eyes, rather than the brown hair and dark green eyes she has in the anime.
Girlfriend of Steel 2nd/Angelic Days
In this series, it is revealed early on that Yui is still alive and working at Nerv in an unspecified capacity. She does not play a major role until the fifth volume, which takes place in the past, when both she and Gendo were junior high students attending the same school. As a junior high student, she is the hard-working class representative with many friends. However, she feels concern for Gendo, and attempts to get him to open up to her. In doing so, she and Gendo manage to become close, but this comes at the cost of her other friendships at the school.
Shinji Ikari Raising Project
In this series, Yui is the head of research operations within Nerv (taking over Ritsuko Akagi's place). Here, Rei becomes her distant relative.
She is shown to be a jovial mother and hardworking scientist, although she does not tolerate too much foolishness. (When Gendo shone his torch in a scary fashion during a blackout, Yui gave him a comical smack.) Yui keeping Gendo "in line" and smacking him for his antics seems to be a running gag in the series.
"Ikari" is "anchor" in Japanese. According to series creator Hideaki Anno, the name Yui was "a simple pun on yui 唯 [only one]." Sadamoto's artbook Der Mond includes concept sketches of several characters; one such sketch for Rei was a dark-haired, green-eyed character named Yui Ichijo.