Wow ACC missed a B-day as well. And speaking of, today's Sumia's Birthday.
So Sumia, your first Peg Knight and flier and almost firmly locked into the Ceada archetype (except for that Knight reclass option). If you're playing on Normal she'd be the one to introduce you to the Pair Up mechanic. Availability-wise she's solid from chapter 3 onward. Class-wise Pegasus Knight is a really good set to have for Lancefaire, Galeforce, and Speed +2 for that extra oomph in doubling. Meanwhile the Knight class tree goes a long way in fixing Sumia's biggest problem, her crappy Str and Def, with Luna, Dual Guard+, and Pavise access. And Cleric access gives her extra healing power from her Staffs, extra magic power from Tomefaire, and extra longevity with Renewal (which supplants her earlier Refresh). She can also make for a great Rallybot with a combination of Rallies Movement, Speed, Defense, Magic, and either Luck or Love. Gameplay-wise, her best bet is to take her up Dark Flier first for early Galeforce then either follow it up with Falcon Knight for a flying healbot and Lancefaire, Great Knight for Luna and better Strength and Defense growth, or Sage to continue building that Magic and starting on that Staff rank. And stat cap modifier-wise, she sports the standard total of +2 that many 1st genners have, but she has the same Speed and Skill modifiers as Panne, the highest of the first gen females not named Avatar, but it comes at great expense to her Strength and Defense stats (although not as badly as Maribelle's Str and Def). Either way though, Sumia cannot pair with Lon'qu meaning that there are no interesting eugenic options for Morgan from her stat-wise.
In fact, her only options are the elusive Chrom, Frederick, Gaius, Henry, and a male Avatar as she serves as Chrom's counterbalance in terms of pairing options for her gender. Preference-wise, her first choice be the Avatar, but everybody wants into his/her pants and Sumia doesn't really bring that much to the table compared to other options (outside of two kids with the Avatar's massive class set) plus the Avatar might not even be the right gender, so barring that her first proper choice is also her last choice, Henry, due to both his access to Dark Mage and both Valkyrie and Thief being decent backups with good skills as well as offering Cynthia a solid 8 classes plus a little something from his Barbarian class tree. Second would be Chrom who would have her as his first choice assuming The Avatar is elsewhere or male, namely because Aether is just that good of a skill and Lucina would be able to get onto the Galeforce (although both children will have sad Defense modifiers). Third is Frederick as his Wyvern Rider class tree and Paladin access is solid and gives her the Aegis/Pavise combo along with Lance and Swordbreaker skills as well as a +2 modifier total added to Cynthia's modifiers. And finally is Gaius who's Myrmidon and Thief class trees provide a solid offensive boost with Swordfaire, Astra, and Lucky Seven access plus an extra skill from either his Fighter class tree, including Hero's Sol or Axebreaker. The reason for my choice in this close race isn't because of what the father offers the child but whether there's a greater demand for them elsewhere, which in Gaius' case, there is with Tharja and Nowi.
The tl;dr is that Sumia can wear many hats due to her diverse class set and solid Speed and Skill modifiers, but her pairing options are ridiculously narrow and she's the most liable to wind up as an old maid, since unlike Chrom, there's no random maiden to shotgun a wedding with.