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), declares to Shinji his affection and love and 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 he eventually does so. Shinji is left broken and distraught by the decision. He is mentioned in the context of Shinji's guilt over killing someone he "loved" in the series' Instrumentality.
In End of Evangelion, Kaworu, as Adam within Rei-Lilith's body, appears before Shinji to calm him and lower his AT barriers in parallel to other characters who saw their most beloved person 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. It is demonstrated that Kaworu represents "love" in Shinji's heart and his own capacity to love others, while Rei represents Shinji's hope for humanity.
KawoShin is the most popular pairing in the Eva fandom, especially in Japan, as demonstrated through number of doujin circles and fanworks, polling, and sheer number of official promotional material and merchandise coupling the two as a romantic pair. The cast largely appears to support the ship, with Rei and Misato's seiyuus claiming the two to be each other's "Orihime and Hikiboshi" live at The Symphony Of Evangelion 1997 event (Kaworu and Shinji are sitting under the stars of Altair and Vega in Evangelion 3.0's poster sharing red earbuds in reference to this legendary couple), and Asuka's seiyuu wishing for the two to "get married" in addition to a happy ending. Kaworu's seiyuu, Akira Ishida, stated in an interview for the Rebuild movies that "the feelings Kaworu has for Ikari Shinji… they won’t change".
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 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. While many have questioned the extent to which Kaworu, as an Angel, actually felt or understood what he claimed to Shinji, both the assumptions that Angels cannot or do not feel as humans/lilin can or that Kaworu was meant to act as a "Jesus" figure have never been confirmed. Some, mainly in the more Western non-Japanese fandom, argue that Kaworu was purposefully manipulating Shinji, in order to further damage him, considering his links to Seele, the main antagonists of the series, though the creator, Hideaki Anno, has claimed he intended for Kaworu to be an "incredibly good person".
The pairing is considered canon by many due in part to their mutual confession and reciprocal affection within the show, in addition to official material which states the two to be romantic interests, such as Hideaki Anno Parano Neon Genesis Evangelion (an official 1997 NGE companion book featuring interviews with Hideaki Anno), which describes Kaworu to be a "same-sex romantic partner/lover" to Shinji. Anno states in an 1996 interview with JUNE magazine, a BL publication, that Kaworu and Shinji's relationship was intended to be homoerotic and released drafts for episode 24 that included Kaworu and Shinji kissing and skinny dipping together, along with illustrations by doujinka Nobi Noibita. Although Anno attributes Akio Satsukawa being mainly responsible for episode 24's "homoeroticism", other storyboard staff have claimed otherwise. In addition, an official anthology published by Khara named "All About Kaworu Nagisa," explicitly claims that Kaworu is the "lover" Shinji lost in End of Evangelion.
Kaworu and KawoShin have proved immensely popular. Many of the spin-offs have expanded on Kaworu's his dynamic with Shinji, often featuring Kaworu as a potential romantic endgame for Shinji. In the official comic parody Petit Eva, Kaworu openly states his desire to marry Shinji, and in Kaworu Nagisa Raising Project, Shinji is Kaworu's only romantic option. 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. KawoShin only increased in popularity with the new Rebuild movies and exploded after Evangelion 3.0, which features this pairing prominently.
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.
Supplemental material links and images:
Tanabata symbolism/references: Q poster, movie (constellation in stargazing scene), 1997 Symphony of Evangelion event (https://www.youtube.com/watch?v=UlwTgRmcdb4), Ladybear icecream collaboration (https://twitter.com/LadyBear_info/status/1280049428271775744)
JUNE Interview: https://17th-angel.tumblr.com/post/62760437979/anno-hideki-long-interview-part-1-i-think-that
JUNE Drafts: https://17th-angel.tumblr.com/post/41710656950/evangelion-episode-24-drafts-complete-translation
NGE Storyboard interview with Higuchi Shinji and Masayuki: https://docs.google.com/document/d/1k4SZdx9nBXC38dD-FJMN4lvJwYhyjAVzRrxIfJuv4xc/edit
Nobi Nobita Ep24 Draft Illustrations: https://www.reddit.com/r/evangelion/comments/jcpmz9/ep_24_draft_artwork_for_kaworu_and_shinji_from/
Another page from JUNE interview: https://tonkatsu-slice.tumblr.com/post/39079430217/for-the-summer-that-will-come-tomorrow-for-the
Kaworu's description page from Hideaki Anno Parano book: https://ibb.co/RbscJxj
Eva store poll: https://ibb.co/tMTD5gC
Top 20 (16th) most popular m/m ships of all time in terms of most doujinshi via pixiv: https://mobile.twitter.com/bem_b3m/status/1327651838062112770
Folder of KawoShin official promo art and merch (including the infamous KISS jewelry collab): https://drive.google.com/drive/folders/16D4LNgTNCZgT8aJkfqgLki_p9pbQwY9i
"All About Kaworu Nagisa" anthology page: https://ibb.co/mX5mDqT
"Who can foresee the future and guess that they can meet again at this moment?
The moment Human Instrumentality Project began, Kaworu’s form appeared in front of him again.
But, is this still the same “him"——
“You have been there all along?”
The question melted in the air. He whispers to Kaworu, as if facing the lover that he is about to lose."
Sadamoto interview for manga: https://tonkatsu-slice.tumblr.com/post/50188846866/can-we-start-this-interview-with-the-secret
NGE2: Another Cases Guidebook page: (https://64.media.tumblr.com/914405a73a68e0d190983626c4efa777/tumblr_mtwvf7qKnN1qeh2rwo1_1280.jpg).
Translation of Kaworu’s character overview:
“While his feelings of love (愛情) for Shinji are strangely strong, his romantic affections (愛情) towards members of the opposite sex such as Ritsuko and Maya are weak. Could this be because he is an Angel? Under these circumstances, forming a ‘rabu rabu’ relationship with Shinji is preferable. As the relationship is satisfactory, concern over forming other relationships is unnecessary.”
And this from the same book, I'm assuming?: https://ibb.co/xmnphqg
Something I see missed/overlooked is according to this blog, the "Shinju" reference in the drafts is mistranslated and of course Evageeks never fixed it: https://tonkatsu-slice.tumblr.com/post/57050782065/in-case-anyone-gives-a-fuck-i-fixed-the. It changes the entire meaning of the scene where people like to say Kaworu "rejects" Shinji. I'm not sure if this matters in the grand scheme of things, but wanted to tell you guys so you know.
Ogata called them friends in the context of 3.0, and interpreted Kaworu's "suki" as like, but still vocally acknowledges the homo interpretation and plays it up. These two links are her pointing out her roles as Uranus (SM), Yukito (CCS), and Shinji (Eva) all have homo elements:
Netflix translations: Kaworu's suki is translated as "like" in ep24 but then translated as "love" in the EoE scene which would go against detractors saying it was intentionally changed to be "more accurate" on Khara's part (https://ibb.co/GvGrxvj)