Robin

"In the beginning of the game, Robin is found lying in an open field by Chrom, her sister Lissa, and her bodyguard Frederick. They find out that Robin is an amnesiac who remembers nothing besides their name, Chrom's name, and his incredible knowledge and experience in fighting battles, which Chrom experiences firsthand after they manage to save a small settlement from a larger group of brigands. Frederick suspects Robin of being a spy due to remembering Chrom's name before their own, but Chrom is convinced of Robin's good intentions and invites them to join the Shepherds as their chief tactician, with the two quickly becoming close friends. Ylisse, Chrom's home country, is then faced with a war against Plegia (which is later revealed to be Robin's home country) and another against Valm two years later, both in which Robin's tactical expertise proves very helpful, especially given Ylisse's limited number of soldiers. There is also the constant threat of the Risen, undead soldiers who terrorize parts of the continent of Ylisse. As the Shepherds continue to fight and recruit more capable members, Lucina, Chrom's daughter from the future, properly introduces herself to them, having only met them while she was disguising herself as Marth. She, alongside the children of other members of the Shepherds, travelled back in time to warn them about the revival of the Fell Dragon Grima after her own world was put on the brink of destruction courtesy of him. Robin, Chrom, Lucina, and the rest of the Shepherds make it their new mission to prevent the Grima of the current timeline from surfacing.

In game, Robin starts in the Tactician class. This class is exclusive to Robin, their children, and DLC characters, and enables them to attack with magic tomes as well as swords. As the avatar character, Robin and their children can switch to any starting class the player desires using a Second Seal (aside from Lord, which is exclusive to Chrom and Lucina), making for one of the biggest skill pools possible for a single character. Robin can also support with every other character (as Kris could previously), and marry any opposite gendered character the player wants, including the future children of other Shepherds that are met via Paralogues (side stories) as more characters within the Shepherds marry each other. Robin's customization focuses more on appearance and even features several options for voice."

Via SmashWiki