ReiShin

ReiShin is the het ship between Rei Ayanami and Shinji Ikari from the Neon Genesis Evangelion fandom.

Canon
Rei and Shinji have a significant relationship thorough the series. They meet in the first Episode and Shinji is intrigued by her mysterious personality. In Episode 6, Rei acts as a human shield to Shinji and he later forces open her entry plug, fearing she might be hurt. Unsure of how to react, Shinji suggests she smiles. Asuka joins the cast in Episode 8, and Rei takes an immediate, and eventually mutual dislike to her. Rei shares a lot less scenes with Shinji in the following episodes as Asuka takes a bigger part in the story. Notably, she sacrifices herself to save Shinji in Episode 23, realizing she has feelings for him. Distraught over her loss, Shinji is overjoyed to find out she is still alive, but she has no memories of her recent past. Eventually, Ritsuko reveals that Rei is in fact part of a series of clones of Shinji's mother Yui, and in fact has three iterations throughout the series. Later in End of Evangelion, Rei joins with Lilith and starts Third Impact, relinquishing control of Instrumentality to Shinji. She is last seen observing Shinji in a beach just before he notices a just-returned Asuka laying by his side.

Fanon
Fans often debate on whether this relationship counts as incest and if Shinji is better off with Rei, whose views on the fandom range from an uncaring, cold "doll" to a caring, warm and philosophical character, or other characters such as Asuka and Kaworu. The fact that Rei is a clone of Yui, Shinji's mother, is sometimes pointed out as motivation for this pairing to be considered incest. Some shippers like to see Shinji's relationship with Rei as sister-like or mother-like.

Detractors also consider Rei as being enabling of Shinji's proclamatic behaviour due to her own passivity and scarcity of her own emotional needs and her almost complete lack of self-worth as a character that is not entirely human, and thus argue she would not develop a healthy relationship with Shinji, particularly contrasted with Asuka.

Reishin used to be very popular in the early days of the fandom in the late 90s and early 00s, but steadily gave way to AsuShin becoming more and more dominant and, more recently after the Rebuilds, is probably in a third place after KawoShin, though it seems to be becoming more popular again. Like those ships, it is featured heavily in official spin-offs, videogames, and manga, as well as doujinshi from those periods. Reishin is a possible pairing in most Evangelion video games such as Shinji Ikari Raising Project and Girflfriend of Steel 2. It is also featured more heavily in spin-off manga such as Angelic Days and Campus Apocalypse, as well as in the manga adaptation of Evangelion.

Owing to Rei's own gargantuan popularity in Japan as one of the most iconic anime characters of all time, Reishin was very popular with a large amount of fan and official material catered to it. Rei and Reishin mostly lost popularity on the fandom over time to Asuka.

In Evangelion 2.0, Shinji gets very close to a much warmer and more open Rei, and many fans believed the movie was going to make it the canon relationship, with the movie ending's going so far as Shinji taking huge risks to save Rei from death, however in 3.0 she was replaced by another clone and Shinji was left alienated from her again. This saw a huge revival in popularity for Reishin after Evangelion 2.0, only to be replaced by Kawoshin after 3.0, which in turn heavily featured that pairing instead.

Fandom
FAN FICTION
 * Rei/Shinji tag on FanFiction.Net