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. Kaworu tells Shinji that he was born to meet him, and that he likes/loves him. However, the next day, Kaworu is revealed to be an Angel.
Hijacking Unit 2, Kaworu attempts to initiate Third Impact. Shinji initially refuses to believe that Kaworu is an Angel, but quickly realizes he must fight him. Shinji defeats Kaworu, and Kaworu asks Shinji to kill him. He tells Shinji that his death will allow humanity to continue on. Shinji hesitates, but he eventually does so. Shinji is left broken and distraught by the decision. He is only briefly mentioned in the context of Shinji's guilt over killing someone he saw as human in the series' Instrumentality and is otherwise absent from it, including the famous Congratulations scene.
In End of Evangelion, Rei takes Kaworu's form to calm Shinji, though he only enters Instrumentality after seeing the image of woman's clutched breasts. Rei and Kaworu, in their forms as Lilith and Adam, discuss with Shinji about the future of humanity and his choice of rejecting Instrumetality.
Like Shinji's other pairings, KawoShin is divisive. The rapid development of their bond, the equally destructive end, and the slash nature took many fans aback.
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, and frequently admit to wishing for a "happily ever after" for the two of them. 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 semi-canon same-sex pairing in a 1995 anime, with many LGBTQ+ fans gravitating towards it, and viewing it as positive representation of themselves, and Kaworu even earning the joking nickname of "Gay Space Jesus" from them.
Meanwhile, detractors often suggest that Shinji was too broken and Kaworu too naive for either to properly understand their feelings. Detractors also argue that portraying Kaworu as the only person who was kind to Shinji is dismissive of many other characters such as Misato and Rei. They point out that Shinji was portrayed as unambiguously straight until Episode 24, and that Kaworu's death damaged Shinji immensely. Many have questioned the extent to which Kaworu, as an Angel, actually felt or understood what he claimed to Shinji, not to mention how the original Japanese uses ambiguous and possibly non-romantic terms. Others argue that Kaworu was purposefully manipulating Shinji, in order to further damage him, considering his links to Seele, the main antagonists of the series. Kaworu also seems to represent a form of idealized, unconditional and self-serving affection that is strongly deconstructed and disapproved of throughout the series.
Nevertheless, Kaworu and KawoShin have proved immensely popular. Many of the spin-offs have expanded on Kaworu's his dynamic with Shinji, much like what happened with ReiShin, although it still tends to have much less space than ReiShin and AsuShin. Most notably, Rebuild of Evangelion 3.0 focused largely on their relationship, albeit with a similar tragic ending, after Evangelion 2.0 focused largely on Rei's own relationship with Shinji. In regards to shipping, the Evangelion fandom is mostly divided between KawoShin and AsuShin, with ReiShin in a third place. KawoShin was fairly popular after the initial release of Episode 24 along with AsuShin and ReiShin but quickly declined in popularity over the years, as in the 2000s the fandom was divided mostly between AsuShin and ReiShin, with KawoShin relegated to more of a niche role. However, it regained some popularity with the new Rebuild movies and exploded after Evangelion 3.0, which features this pairing proeminently, similar to how 2.0 did with ReiShin. As of recent years, however, it seems to be declining again, as AsuShin has remained popular regardless and ReiShin is becoming more common again.
As with other ships, Kawoshin gained another popularity spike after the 2019 Netflix release of Evangelion. 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. See Kaworu's lines in Episode 24 on EvaWiki