Marthcina

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

Fire Emblem series
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 "Marth". Later, however, she would reveal his true identity to his father.

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 as an Echo Fighter. 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.

Fanon
This was one of the impossible shippings, since in the Fire Emblem series, Lucina never meets Marth personally due to her being a descendant of him, but at least she knows him in the legends.

However, it has become controversial when Lucina was introduced as an Echo Fighter for Marth in the Super Smash Bros. series, as both can be selected. Fans who are not familiar with Fire Emblem and Lucina's true origin pair her with Marth as they both have the same attacks.