Zelmus

Zelmus is the ship between Zelda and Samus Aran from the Super Smash Bros. fandom.

Canon
Zelda and Samus are both playable characters, with Zelda being added in Melee, while Samus was playable since the very first game. Within the series, they are both usually depicted as blonde (the Twilight Princess version of Zelda first seen in Brawl is confirmed to be dark blonde) and blue-eyed.

Super Smash Bros. Melee
In Event 15, "Girl Power", the player character has to defeat Samus and Zelda (along with Princess Peach) all while being shrunk. In Event 20, "All-Star Match 2", the player has to once again defeat both of them (alongside Link, Captain Falcon and Fox) in a time limit.

Super Smash Bros. Brawl
This is the game that gave Samus the ability to switch between her Varia and Zero Suit, which was most likely inspired by Zelda's ability to turn into Sheik at will in Melee. Besides the ability to change forms in general, Zero Suit Samus is very much the Sheik to Samus' Zelda: The Zero Suit allows the usually slow, but powerful Samus to become a lot more agile, much like how losing the dress as Sheik gives Zelda far more mobility. Additionally, Zelda's more stoic personality as her Twilight Princess self is very similar to Samus.

Then there's the "Unwanted Suitors" event, where Zero Suit Samus and Zelda fight off an invisible horde of Captain Falcons and Luigis.

The Subspace Emissary
Though they are both major characters in the story, the only time Samus and Zelda ever cross paths is when they enter Subspace alongside the majority of other fighters and end up getting turned into trophies by Tabuu. During the main story, Zelda was with Peach and Samus was with Pikachu, everyone gets rescued by Luigi, Ness, King Dedede and Kirby, allowing for the entire cast to face off against Tabuu together.

Super Smash Bros. for Wii U and 3DS
Right after Brawl gave her the ability to change form, both Samus and Zelda had to give it up due to the 3DS struggling with transforming characters, leading to Samus, Zero Suit Samus, Zelda and Sheik being completely separate characters. As a compromise, Sheik is given a copy of Samus' Down-Special, which they retained in Ultimate.

There's the "Identity Crisis" event match, where Sheik and Zelda have to fight Samus and Zero Suit Samus.

Super Smash Bros. Ultimate
Zelda's Ultimate redesign gave her a more outgoing and friendly personality, though her hair is once again a similar shade to Samus'.

In the World of Light opening cinematic, Samus and Zelda are opposing Galeem alongside most of the other fighters, with Zelda telling her allies to "stow their fear". When Galeem attacks, both of them get disintegrated and end up trapped as Spirit templates until Kirby sets out to rescue them.

Fanon
Zelmus is the most popular femslash ship in the Super Smash Bros. fandom, along with being the most popular ship for both characters (Zelink doesn't count). Both variations of Samus/Zelda and Samus/Sheik are generally agreed to be the most common femslash ships in the fandom. Whether Zelda and Sheik can be considered the same person or not is a hotly debated topic in The Legend of Zelda fandom.

Even though both characters have been there since Melee, it took until Brawl for the ship to get mainstream attention. People were drawn to Zelda's personality change and argued that she would make a great match for Samus' personality. There's a bit of a rivalry with SamuPichi and ZelPeach, as all three of them are female veterans with similar designs but distinct personalities, leading to fans getting into fights about which variation they prefer. Strangely, just shipping all three of them in a polycule is rarely seen.

Though the pairing is still very popular after the release of Ultimate, shipping that version of Zelda with Samus is not as common as her Twilight Princess version. While fans love the Ultimate Zelda, they found her previous incarnation to be a far better match for Samus, or think that that variation of the ship is too similar to SamuPichi.

Fandom
FAN FICTION