As the first aerial unit recruited, Sumia can be very useful utility early on. She can be paired up with a unit of lesser movement, like Chrom or the Avatar, and bring them to their needed destination. She also gives large bonuses in skill and speed. In terms of her stats, she gains skill, HP, and speed quickly, with her other stats tagging along. Pairing her up with either the Avatar or Chrom can highlight her potential, as she can allow them to double and finish off opponents. She also gets a small increase in firepower, as she has neither high strength nor magic ratings. Sumia is also fragile, especially to bow users, which can be a pain when trying to carry a unit. Despite these flaws, Sumia is not a unit to be overlooked.
In contrast to Cordelia, Sumia's growth rate and stat caps in magic are higher, which makes her a better Falcon Knight healer or a Dark Flier mage. However, the growth is still quite low and is lower than her strength growths. Her higher speed and skill will also mean that she will double attack and critical much more often. Sumia should spend some time in the Dark Flier class regardless of the player's final class for her since she can learn the useful Galeforce skill. It is also important to note that her magic caps in the Dark Flier class are higher than her strength caps as a Falcon Knight. Nevertheless, Sumia as a Falcon Knight has its perks of slightly higher speed and skill caps over Dark Flier. Spending some time in the Falcon Knight class is not a bad idea either, since Lancefaire gives added strength when Sumia uses a lance to attack.
Sumia's two class options are Cleric and Knight. As a Cleric, she may lose her ability to attack but she learns Miracle, which given her high luck, she can survive battles in dire situations. Healtouch also works well if the player decides to make her a Falcon Knight healer. As a War Cleric, Sumia is as strong as Lissa, but is faster and has higher skill. The biggest gift from this class is Renewal, which should be on her regardless of her ultimate class since Relief alone is not useful in tight gameplay. Sage wise, Sumia is not as stellar as Miriel or Ricken, and the skill to pull from this class would be Tomefaire to boost her magic offensive as a Dark Flier.
Sumia as a Knight makes for a very interesting unit. She is much faster than Kellam and has more skill, leading to more double attacks and criticals. Sumia does not learn many skills worth while as a Knight, but as a General and Great Knight, things are different. The General's Pavise will shave some damage off of her low defense caps in her base classes and with her high skill, will activate it often. Great Knight will teach her Luna making her a deadlier force offensively and Dual Guard+ will make her an even better Pair Up partner.