Marthcina

Marthcina is the family ship between Marth and Lucina from the Fire Emblem fandom.

Fire Emblem
In Fire Emblem: Awakening, Lucina is a distant descendant of Marth and Caeda after they married centuries ago. However, in order to surprisingly help her father in the alternate world, she decides to hide her true identity by putting on her mask and pretending to be a man named "Marth". When questioned by her father, she states that she is not Marth, rather, named after him. Later, however, she would reveal his true identity to his father. In Fire Emblem Warriors, she states that she took Marth's name because he is her symbol of hope.

Super Smash Bros. series
In Super Smash Bros. Melee, Marth was the first Fire Emblem character added to the fighter roster. Later, in Super Smash Bros. 3DS / Wii-U, Lucina would be added, closely mimicking his playstyle. In Lucina's introduction trailer and Roy's trailer, Lucina and male Robin face off against Marth and Ike. Since then Marth and Lucina can be selected from that version. In the release of Super Smash Bros. Ultimate, Lucina is added as one of Marth's "echo fighters", or characters that play so similarly to another that they are counted as almost the same character.

Fire Emblem Warriors
In Fire Emblem Warriors, Lucina shows a lot of respect to Marth. Lucina speaks to Marth the legends about him, however, Marth doesn't see the significance in them or himself. Marth even says that he is uncomfortable with how much Lucina praises him. When Marth questions Lucina on why she took his name, she states that she did it because he is her symbol of hope. She also says that taking her name is what caused her bravery.

Fanon
This was one of the impossible shippings, since in the Fire Emblem series, there are only three instances where they meet, none of which are canon. Regardless, some people ship it in a non-romantic way due to Lucina's admiration for Marth. Due to Lucina's status of an echo fighter as Marth in Super Smash Bros. Ultimate, people who don't play Fire Emblem or are new to the series tend to ship it romantically. There are not many fan work of the pairing on social media, as AO3 currently contains less than 10 fanfictions of the pairing. Due to it being an incest ship, romantic pairings of the ship is heavily disliked.