ReiUsa

ReiUsa is the femslash ship between Usagi Tsukino and Rei Hino from the Sailor Moon fandom.

Manga
Ami tells Usagi of a very pretty girl that she sees on her bus ride home. Intrigued, she joins Ami and is taken aback when she sees her. She is so infatuated, that she gets off when the girl does. Luna asks her if you should be really doing this, but Usagi says the girl is so pretty and she has time to kill, with hearts in her eyes. Usagi loses sight of the girl, and gets angry at Luna for it. When Usagi walks into Hikawa Shrine, some crows attack her, causing the girl, Rei, to come out and use a demon ward on her, causing her to collapse. Rei apologises. Usagi is a little surprised, but since she is such a pretty girl she doesn't mind.

Later Rei attempts a fire reading to what is behind the mysterious disappearing buses. She sees a vision of Usagi being strangled, and becomes concerned that Usagi may be in danger. Soon Rei is captured by Jadeite, so Sailor Moon uses her tiara to free her. Rei awakens as Sailor Mars, and finishes off Jadeite. Usagi is elated that Rei is a Sailor Guardian and so happy they found her. Afterwards, the two end up becoming close friends.

90's Anime
Usagi visits Hikawa Shrine with Naru to buy an amulet. After Rei appears to chastise her Grandpa for hitting on the girls, Usagi remarks that she looks so cool and pretty, while blushing. Usagi also says that she wouldn't mind being a shrine maiden part-time, with a big grin on her face. Rei then senses an unusual aura and slaps a demon ward on Usagi's forehead, causing her to faint. Rei sheepishly says that's not what was supposed to happen. Rei gets Usagi a place to rest, and when she awakens, apologises.

The next day, Usagi visits Rei again and tells her she saw the Demon 6:00 bus. Rei immediately misinterprets that as an accusation toward her and her Grandpa and asks Usagi to leave. Usagi says she's sorry and tells her she just wanted to be friends. Usagi then makes it on the Demon bus to investigate, and travels to another dimension by the enemy. Rei also gets sucked into a black hole to the same place as Usagi. Usagi transforms and saves Rei from the monster holding her captive. After Rei awakens as Sailor Mars and things are resolved, the two become friends and hang out together.

Rei quickly becomes interested in Mamoru and manages to go on a date with him. Usagi decides Rei shouldn't be with Mamoru, that it's dangerous, so she must follow them, although Luna knows Usagi is jealous.

The 5 Senshi decide to go to a ski resort for a vacation. Usagi is interested in the Moon Princess Contest, which Rei taunts whether she'll even be good enough for it. This sparks Usagi and she confidently exclaims she can do the course. Rei also participates, as she has very good skiing ability. At the start gun, Rei zooms off, but Usagi stalls nervously, and is sent flying down the slope by a push from Yuuichirou. Since the Contest was a trap devised by the Dark Kingdom, the other skiers wipe out until only Rei and Usagi are left. Usagi eventually passes Rei, who then takes that as a challenge to beat her. The enemy appears, and forces them to ski into a crevice, and then a dead-end. While they're trapped in there, Usagi brings out Mamoru's music box to cheer her up. She then remembers Rei likes Mamoru and apologises. Rei says it's okay, because she's over it now. She knows that it was only ever a one-sided crush, and Mamoru loves Usagi. She won't chase after Mamoru because she knows the two of them are meant for each other.

Later Usagi convinces her friends to go along with her plan of them pretending to fight in order for the Dark Kingdom to take interest and take her to their lair so she can save Mamoru. Usagi complains that Rei is too harsh on her, but Rei retorts that they have to make it look believable. When the night comes for Sailor Moon to meet with Kunzite, the others wait in the bushes. Usagi is quickly attacked and choked by a youma, and the Senshi argue over whether to go and save her yet. Mars is the most set in staying put, which makes the other three question if maybe she actually hates Usagi. Rei gets angry and tells them that if she did, she wouldn't have held onto Usagi's most important item, the Moon Stick and Silver Crystal for her.

When the Sailor Senshi are preparing to fight the Dark Kingdom at the North Pole, Usagi tells Rei she should kiss Yuuichirou before they leave, just in case. Rei says nothing bad will happen to them, but when they get there, the Senshi are picked off one-by-one by Queen Beryl's DD Girls. After every death Rei tells Usagi she can't cry, and she has to be strong. Eventually it is only Rei left, and she sees the Youma she must fight. Usagi cries in her arms because she doesn't want Rei to die. Rei says she never said she's going to die, anyway, and promises to take care of the monster real quick. Rei is trapped in an ice chamber by one, but is able to kill it. The second one brings out her presumably dead body and Usagi watches blankly. Rei isn't done yet, though, and with her last strength, she uses Fire Soul and kills the last one.

One day Rei has a nightmare of the world falling to ruin, including the Sailor Senshi crumbling to dust. Rei, whose mind is on what she dreamed, doesn't look where she's going and walks right into Usagi, hurting both their heads. They take turns rinsing their foreheads on an outside tap. As Rei walks away, she asks Usagi if she has any dreams. The two then lean against a railing and talk together.

90's Anime

 * Episode 11: When the group splits up to investigate, Luna puts Usagi with Rei.
 * Episode 12: Rei wins two tickets to a luxury romantic cruise, so Usagi asks if she can go with her.
 * Episode 24: Sailor Mars coats Sailor Moon's Moon Tiara Action with her Fire Soul to finish off the enemy of the episode.
 * Episode 63: Usagi says Rei is really cool, then Rei spills hot water and burns herself. Minako asks what's wrong, but Rei doesn't answer.
 * Episode 95: When Usagi immediately thinks of Rei as someone she can rely on for all romance problems and goes over to her place

Fanon
The ship sailed as a result of the two being main characters, spending a vast amount of time together and canonically being good friends and caring a lot about each other.

Rei and Usagi's constant bickering also intrigued the shippers. Especially in the anime, where their rivalry was much more heated. Their bond was made even stronger with the underlying respect and commitment to protect Usagi that Rei had. Even though they fight all the time, that's what good friends do and they understand each other. Fans believe that Usagi entrusted her Moon Stick to Rei because she would be the least likely to give in if Usagi was endangered during their plan.

ReiUsa is a fairly popular ship in the fandom. It rivals the ships Reinako, Reideite, SeiUsa and MamoUsa. On AO3, it is both Usagi's and Rei's 3rd most popular ship.

Fandom
FAN FICTION
 * ReiUsa tag on Fanfiction.net
 * ReiUsa tag on Fanfiction.net
 * ReiUsa tag on Fanfiction.net

TUMBLR
 * Flame Bunnies | Usagi x Rei blog
 * Flame Bunnies | Usagi x Rei blog
 * Flame Bunnies | Usagi x Rei blog
 * Flame Bunnies | Usagi x Rei blog
 * Flame Bunnies | Usagi x Rei blog
 * Flame Bunnies | Usagi x Rei blog

DEVIANTART

INSTAGRAM

PIXIV

Trivia

 * Both their Sailor names start with "M" (Moon and Mars).
 * According to multiple accounts, series director Kuniko Ikuhara shipped Usagi and Rei.

Variations

 * AmiReiUsa refers to the ship between Rei, Usagi, and Ami Mizuno.
 * Inner Senshi refers to the ship between Rei, Usagi, Ami Mizuno, Makoto Kino, and Minako Aino.
 * ReiUsaMina refers to the ship between Rei, Usagi, and Minako Aino.
 * Sailor Senshi refers to the ship between Rei, Usagi, Ami Mizuno, Makoto Kino, Minako Aino, Chibiusa Tsukino, Haruka Tenou, Michiru Kaiou, Setsuna Meiou, and Hotaru Tomoe.