Morgan is one of the second-generation characters that appears in Fire Emblem: Awakening and the Avatar's child. Morgan's gender is the opposite of the Avatar's own gender, and thus has both a male and female appearance. Whatever gender Morgan is, (s)he strives to be an accomplished tactician just like the Avatar, as evidenced by their support conversations together.
Similar to Lucina, Morgan can have siblings depending on who the Avatar married, or can be an only child. Because the Avatar has a larger number of possible marriage options than the game's other characters, Morgan's list of potential siblings is also quite expansive. They include Lucina, Kjelle, Laurent, Severa, Gerome, Noire, Cynthia, Inigo, Owain, Brady, Yarne, and Nah.
Morgan's starting class is the base class of the Avatar's spouse, unless the spouse's class is unique. In this case, Morgan will instead be a Tactician, like the Avatar.