Metacina

Metacina is the het ship between Meta Knight and Lucina from the Super Smash Bros. fandom

Canon
Meta Knight and Lucina are both swordfighters who wear or have worn masks that conceal their faces, wear capes. Both are also similar in design to major characters in their franchises, with Lucina having a heavy resemblance to her ancestor, the Hero King Marth and Meta Knight looking similar to Kirby albeit with a blue color and wings.

In terms of personality, both are depicted as outwardly no-nonsense, serious and often comically serious due to their tendency to not joke around as much as their peers or engage in less serious talks, with some hidden sides of themselves that indicate a much more easygoing persona beneath the serious one.

Fanon
The main basis for the ship's dynamic is the idea of Lucina finding Meta Knight adorable and taking every chance she can to admire him, much to the Knight's protests and initial discomfort. While some works have Meta Knight react with apathy or disdain at Lucina's admiration, others have him slowly warming up to her to the point that he won't make an effort to avoid her. More recently they have been shipped on a more traditional romantic knight/princess dynamic, with Meta Knight acting as Lucina's close protector and both respecting each other's cred and skill as warriors.

Another dynamic that might come into play is Meta Knight being insecure about his feelings for Lucina, often based off his implied insecurities in canon over his face and skills, with Lucina being more confident about starting a relationship with the Knight if she isn't written to be as insecure as him. Occasionally Chrom may be written as being disapproving of their relationship to add more dynamics to the narrative.

Part of the ship's dynamic was inspired by Meta Knight teaming up with the Fire Emblem heroes Ike and Marth during the story mode of Super Smash Bros: Brawl: The Subspace Emissary, with Lucina's resemblance to Marth in both appearance and moveset giving the fans ideas about Meta Knight teaming up with other Fire Emblem heroes, Lucina included.

While the ship can be popular in certain circles, it is quite small in amount of fanfiction and fanart compared to other more common Smash Bros ships; it has no specific fanfictions in AO3 with 2 fanfics having it as a minor ship in the story, and about 3 fanfictions in Fanfic.net; there are more fanworks to be found in sites such as Pixiv with a record of 22 novels under the Rukimeta (ルキメタ) tag.

FAN FICTION

 * Meta Knight/Lucina tag on AO3
 * Meta Knight/Lucina on Fanfic.Net
 * Meta Knight x Lucina stories on Wattpad

TUMBLR

 * Metalucy posts on Tumblr
 * Meta Knight/Lucina posts on Tumblr

TWITTER

 * Rukimeta tags on Twitter
 * Meta Knight/Lucina tags on Twitter

ART

 * Rukimeta tag on Pixiv
 * Metalucy tag on DeviantArt

Trivia

 * Meta Knight and Lucina are shown side-by-side in the official Super Smash Bros Ultimate panoramic art.
 * The name "Metacina" comes from joining Meta Knight's and Lucina's name. The same rule applies to Metalucy and the ship's japanese name where Lucina's name is spelled as "Rukina" with the order of the name combination changed.