In Episode 24 of the series, Kaworu has only brief encounters with Rei. He empathizes with their shared situation as Angels embedded into human cloned bodies, saying she's the same as her. This confounds Rei, however. Later, when Shinji is about to kill Kaworu, Rei discreetly watches, and Kaworu smiles at her.
In End of Evangelion, Kaworu and Rei are fused as Adam and Lilith and enact Third Impact. They together converse with Shinji regarding his life and the human condition.
KawoRei is a popular pairing, but not as frequently talked about as the main three Shinji ships in NGE fandom. It has a significant amount of fan material in the form of fanart and doujins, as well as official material. Not unlike MisaKaji, another non-Shinji ship, KawoRei is commonly featured in AsuShin ships as a secondary relationship, and is quite popular with these shippers and Asuka fans who also wish to see Rei paired with someone. This also means that proportionally, standalone KawoRei works aren't too common. Naturally, ReiShin and KawoShin shippers see it as a rival ship, and OT3s in Eva fandom are quite rare, with AsuReiShin being much more popular than AsuKawoShin, with AsuKawoRei being even rarer. KawoRei became significantly more popular in the late 2000s with the advent of the Rebuild movies, as Rei used to be paired instead with Kensuke Aida in AsuShin fics. Another rival ship is AsuKawo, which was elevated to a more equal footing with the release of 3.0 but gradually lost popularity again. Both are stereotypically popular with yumejoshi and Kaworu and/or Rei fans that dislike Shinji or simply don't ship him with them.
Supporters like KawoRei for its shared "Angelic" aesthetics and their similarities as Angel hybrids. They argue the story inevitably pairs them together in EoE by having them both fused as Adam and Lilith. Their potential dynamics borne out of Kaworu and Rei's shared lack of social sensibilities are seen as very appealing, especially for comedic works. Detractors consider it to have no basis in canon whatsoever and them as being completely incompatible with each other, or too interested in Shinji instead. Some consider it incest.
Despite its relatively little screen time in canon, KawoRei is one of the most popular pairings in extracanonical material like games and spin-off manga, as well as merchandise and official art, unlike other non-Shinji ships like AsuKawo. Kaworu is otherwise only shown to be interested in Shinji in these but has significant teasing regarding Rei, and suggestively enough, in the Shinji Ikari Raising Project manga, Kaworu declares Rei is "the only woman I have eyes for", and comments on her as "cute" in some games. He also shows great concern for her in other spin-offs like Campus Apocalypse. The Girlfriend of Steel 2 game also suggests they get together if Shinji pursues the Asuka route. The Evangelion manga adaptation has Kaworu receive Rei's feelings for Shinji through the Angel Armisael. In the Rebuild of Evangelion series, adult versions of Kaworu and Rei are seen talking to each other in the rebuilt world in the ending, not acknowledging Shinji. This has been seen by KawoRei shippers as suggestive for canonicity of this pairing in the Rebuild continuity. This has also led to a resurgence in its popularity post 3.0+1.0.