Inner Senshi

Inner Senshi is the friendship between Usagi Tsukino, Ami Mizuno, Rei Hino, Makoto Kino, and Minako Aino from the Sailor Moon fandom.

Canon
During the Silver Millennium, Sailor Mercury, Sailor Mars, Sailor Jupiter, and Sailor Venus were guardians to Princess Serenity. After the Moon Kingdom fell, they were reincarnated in the late 20th century with civilian identities. Eventually each of the five awakened their planetary abilities, and joined together as a result. They almost always fight together. If one of them starts out fighting a monster alone, it isn't long before the others join in. Even into the 30th Century, Mercury, Mars, Jupiter and Venus remain guardians to Sailor Moon, who becomes queen.

90's Anime
Usagi comes up with a plan to trick the Dark Kingdom into letting her save Mamoru. She manages to convince the other four to fight with her in order to convince the Dark Kingdom that she wants to join them. They are very serious about training, Mars saying they have to make it look believable that they were having a fallout. The time comes to confront Kunzite. Sailor Moon is alone waiting for them, but Mercury, Mars, Jupiter, and Venus are close by in hiding, watching. Unfortunately, Sailor Moon's plan doesn't go as intended and she gets choked by Kunzite's youma. This causes the senshi to argue on whether they should go and help her yet. Mars is the most set on staying put, making the others question if she actually hates Sailor Moon. She says that if she did, she wouldn't be holding onto her most valuable item, the Moon Stick, for her. The four of them then decide to bust out and save Sailor Moon.

Afterwards, the five senshi prepare to confront the Dark Kingdom. They perform a "Sailor Teleport" that transports them to the North Pole. Beryl's most powerful Youma, the DD Girls appear to defeat the senshi. One by one, the senshi are killed off, until only Sailor Moon is left, but not without eliminating each of the Youma first. Jupiter, Mercury, Venus and Mars each gave their all so Sailor Moon can still live. After every death, Sailor Moon is more and more devastated, but with encouragement from their spirits, she finds the resolve to keep going.

Sailor Moon is transported into the Dark Castle by Beryl, and after a struggle, she is able to cure Endymion from his brainwashing. She is tempted to kiss him, but she stops herself because she knows none of her friends got to kiss the boy they liked. Sailor Moon confronts Beryl, who had now merged with Metalia, with her Moon Stick. She transforms into Princess Serenity, and all the spirits of Mercury, Mars, Jupiter and Venus appear around her, holding onto the Moon Stick, and invoke their power into it.

Super Beryl is defeated, and by a miracle, the five senshi are reborn right back into their previous lives, but with no memories of their friendship together. The appearance of a new enemy requires Luna to reawaken their memories and Sailor identities, and they immediately go back to being their lively friend group.

Fanon
The ship naturally formed as a result of the five being the main characters, canonically being best friends and spending lots of time with each other. It is usually shipped as a friendship, and occasionally as a poly ship.

The term "Inner Senshi" technically refers only to Sailor Mercury, Mars, Jupiter and Venus, the guardians of Princess Serenity, but Sailor Moon is most often included as well. This is likely due to the fact the Moon is inside the Inner Solar System, and while being the person the other four protect, she is also a Senshi. Jupiter is actually outside the Inner Solar System, but is included because "Inner Senshi" is mostly a fan-coined term to differentiate the first five soldiers from Sailor Uranus, Neptune, Pluto and Saturn (who are commonly called the Outer Senshi). Chibiusa (Sailor Chibimoon) is also sometimes included in the Inner Senshi.

Fandom
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Variations

 * AmiReiUsa refers to the ship between Ami, Rei, and Usagi
 * AmiRei refers to the ship between Ami and Rei
 * AmiUsa refers to the ship between Ami and Usagi
 * MakoAmi refers to the ship between Makoto and Ami
 * MakoMina refers to the ship between Makoto and Minako
 * MakoRei refers to the ship between Makoto and Rei
 * Minami refers to the ship between Minako and Ami
 * Reinako refers to the ship between Rei and Minako
 * ReiUsa refers to the ship between Rei and Usagi
 * UsaMako refers to the ship between Usagi and Makoto
 * UsaMina refers to the ship between Usagi and Minako