To be fair, I didn't specify that my request wasn't for a Frederick costume.
That said, Sully's B-Day today.
She's the first of the Christmas Knights and the Skill/Speed oriented one (inverting the role for the first time since FE8). Disregarding the oddity of Tellus' Knights, she's the first female Cavalier since Cecil from FE3 (although FE8's Amelia can promote into one, but why would you when she can get HUGE!!!; and if you decide to not disregard the Tellus games then FE10's Fiora comes the closest). Though if you include Paladins then you've also got Midia, Matilda, Midia again, Midia yet again, Amalda, Isadora, Titania, and finally Midia twice more. Anyways, Discipline is actually one of the better base skills early on since you can build weapon ranks twice as fast and get out of E rank that much sooner. Additionally since Sully starts as a Cavalier, she'll have two separate weapon ranks to raise at once, making her transition to either Paladin, Great Knight, or her Myrmidon classes all the easier (she does need to spend time as either a Great Knight or reclass from a promoted form to get to Wyvern Lord so that she isn't dependent on a Bronze Axe for offense in either Wyvern Rider or Griffin Rider). She also gets Luna, Astra, Leathality, and Swordfaire for offensive boosts (plus Strength+2 and technically Defender, both of which are kinda minor), Avoid+10, Outdoor Fighter, Quick Burn, and Sword and Lancebreakers for additional avoid (and Tantivy which is too situational and generally not the best use of a skill slot unless you have nothing better to add), Aegis, Dual Guard+, Vantage, and Defender for defense, and Deliverer and Pass for movement boosts.
As for her stat modifiers, her's are set to match her role as the speedy Christmas Knight with high Skill and Speed at the expense of Strength, Magic, and Defense, leaving her with a minor +1 to her overall stats. For the purposes of maximizing Morgan's stats given an appropriately stat distributed male Avatar, a Lon'qu sired Kjelle can birth a Morgan with the highest Skill modifier of +10 and either a Frederick or Vaike sired Kjelle can birth a Morgan with a -5 to Magic. As for potential mates, since Kjelle automatically gains the Knight class tree out of nowhere she'll always have more than the basic three class trees to choose from. Unfortunately, this also means that most potential fathers will overlap with one or more of her potential class sets, especially Lon'qu who's only contributions are Thief and Trickster and Frederick who passes nothing but a shitty Magic stat cap modifier and a free skill that all kids get from their fathers. Her best pairing option is Donnel, since despite both of their mediocre modifiers, he's her only pairing that offers the full three class trees to Kjelle, including the oft sought after Pegasus Knight class tree and the highly useful Mercenary class tree plus the Troubadour class tree offers Renewal and Dual Support+ for even greater defensive options and one of his Fighter/Warrior skills or Aptitude. Her next best are Henry for Dark Magic and having the second highest class contribution of eight classes, followed by Kellam for his high Defense modifier and five classes (plus a free donation of Pavise to save Kjelle from having to spend time as a General), and finally Gregor for his physical oriented modifiers and the Mercenary class tree. Incidentally, because Kjelle gets so many great physically oriented classes for free, Sully will probably gain the least mileage out of the Avatar outside of one of the non-parent characters (although the Avatar's modifiers will suck less than Donnel's does). Additionally, out of all of Chrom's potential brides, Sully is the worst outside of defaulting onto the Maiden since Kjelle will only inherit his lame Archer class tree and Aether while Lucina will miss out on getting onto the Galeforce which only happens with Sully or the Maiden as her mother (although she does get Astra as a consolation prize, but Olivia and the Avatar can get her that too).
Ultimately, despite her modifiers suggesting otherwise, Sully is probably the most physically oriented first gen female outside of the Avatar since unlike Cherche, she's not a staffbot sitting on top of a Wyvern, and unlike Panne and Nowi doesn't need stones to transform (in fact, Sully is the only first gen female without a natural magic or staff using class with Bride being her only staff option, and yes, this includes ladies like Flavia who like Panne has Trickster letting use Staffs). Plus her daughter is fairly unscrewable in terms of inheritance due to her automatic head start and even higher natural emphasis on physical classes.