Kaworu only appears in Episode 24 and the End of Evangelion. By the time they meet, Shinji feels completely alone. His friends have moved away, Tokyo-3 has been destroyed, he has lost all hope of earning his father's approval, he is afraid of Rei and Misato, and he is unsure of where Asuka is or how to connect with her.
Kaworu is friendly, gentle, and open towards Shinji, and Shinji, quickly opens up to him in turn, even blushing when the two first meet. The two take a bath together where Kaworu holds Shinji's hand (an action which Shinji does not reject) and tells Shinji he deserves affection and that he likes/loves him. Later that night when the two sleep in the same room, tells Shinji that he believes he was born to meet him. However, the next day, Kaworu is revealed to be the Seventeenth Angel.
Hijacking Unit 2, Kaworu breaks through Terminal Dogma, supposedly attempting to initiate Third Impact. Shinji initially refuses to believe that Kaworu is an Angel, but quickly realizes he must stop him. Shinji defeats Kaworu, and Kaworu, saying he was glad to have met Shinji, asks Shinji to kill him in order to allow humanity to continue on. Shinji hesitates, but eventually does so. The episode ends with Shinji left broken and distraught by his decision, he tells Misato that Kaworu was the first person to ever say he liked/loved him and Shinji says he liked/loved him too. He is also mentioned again in episode 25 in the context of Shinji's guilt over killing someone he both saw as human. In the Japanese version, it's also said that Kaworu was someone he liked.
In End of Evangelion, Kaworu, as Adam within Rei-Lilith's body, appears before Shinji to calm him before entering Instrumentality. Rei and Kaworu, in their forms as Lilith and Adam, discuss with Shinji about the future of humanity and his choice of rejecting Instrumentality.
Like Shinji's other pairings, KawoShin has proven incredibly divisive and controversial, with strong fans and strong detractors.
KawoShin's supporters often state they were drawn in by Kaworu's kindness towards Shinji, with many viewing the pairing as a potential means for Shinji to begin opening up to others more. They view Kaworu as his most healthy potential relationship as a result of this. In addition, this wish often results in many enjoying it as a "tragic" pairing of sorts, with plenty of fanwork within the Angst genre. Many Kawoshin fans also praise the pairing due to its nature as a same-sex pairing in a 1995 anime, with many LGBTQ+ fans gravitating towards it, and viewing it as positive representation of themselves. Kaworu has earned the joking nickname of "Gay Space Jesus" from them. Supporters of the pairing occasionally accuse detractors of being biphobic or homophobic. These fans consider the pairing canon due in part to what they see as a mutual confession and reciprocal affection within the show. They point to some material such as Hideaki Anno Parano, an interview and essay collection with Anno and others, in which the editor's character guide describes Kaworu to be a "same-sex love interest". Anno talks about their relationship in an 1996 interview with JUNE magazine, with a scriptwriter's early drafts for episode 24 that included Kaworu and Shinji kissing and skinny dipping together, something he eventually rejected. Supplemental material however only describes them as friends. Some cast members seem to like the ship, while others have denied it.
Meanwhile, detractors often suggest that Shinji was too broken and Kaworu too naive for either to properly understand their feelings. They point out that Shinji never showed romantic or sexual interest in men prior to Ep 24, thus suggesting his attraction may have been more a symptom of his desperation as opposed to closet bisexuality. Furthermore, other characters were also kind to Shinji - such as Misato - and Kaworu's death damaged Shinji immensely, putting doubt on the suggestion that their relationship was, or could have been, healthy. Many have questioned the extent to which Kaworu, as an Angel, actually felt or understood what he claimed to Shinji. Some further argue that Kaworu was purposefully manipulating Shinji, in order to further damage him. Kaworu seems to represent a form of idealized, unconditional and self-serving affection that is deconstructed throughout the series. However, series creator Hideaki Anno, has claimed he intended for Kaworu to be an "incredibly good person".
Nonetheless, Kaworu and KawoShin have proved immensely popular. While there is evidence in Kaworu's lack of appearance in the TV ending of Evangelion that he was intended as a one-shot character, some spin-offs have expanded on Kaworu and his dynamic with Shinji, with a large amount material of fanwork dedicated to it. In the comic parody Petit Eva, Kaworu openly states his desire to marry Shinji, and in Shiinji Ikari Raising Project, Shinji is Kaworu's only romantic option. In Girlfriend of Steel 2, he is one of the three romantic routes for Shinji. This also applies to the other popular Evangelion pairings, ReiShin and AsuShin, which also enjoy a large amount of fan and licenced material. Most notably, Evangelion 3.0 focused largely on their relationship, albeit with a similar tragic ending.
As with other ships, Kawoshin gained another popularity spike after the 2019 Netflix release of Evangelion. However, the decision to translate "suki" as "like" as opposed to "love" as in the DVD translation had sparked controversy among Kawoshin shippers and non-shippers alike. Many fans, especially LGBTQ+ fans, felt as though the new translation censored the nature of their relationship. On the other hand, other fans argue that there is evidence that Khara attempted to bring the dialogue closer to the original Japanese script, therefore making the backlash unwarranted.