Leaf's stats/permissions have the really basic version of his abilities, but because I enjoy nerding over game mechanics, I'm going into more detail here.
Data here is taken from Seisen no Keifu; Leaf differs in power between the two games, but since Thracia 776 is first chronologically, I think it's easy enough to handwave the difference by saying that he took a level in badass once he hooked up with Celice's army.
STAT GROWTHS:
HP: 130%
Strength: 60%
Magic: 7%
Skill: 50%
Speed: 45%
Luck: 25%
Defense: 50%
Magic defense: 10%
When discussing a character's badassery in Fire Emblem, stat growths are your bread and butter. We live and die by the RNG, unlike many games where you're guaranteed certain stat-ups (or at least a certain amount of stat-ups in variable places) when a level is gained; in FE, the game pulls a random number for each stat, and if it's lower than the growth rate, you gain a point in that stat. This means it's possible (though unlikely) to gain in everything on a level up, and also that it's possible (and usually happens at the worst possible time) to gain no stats at all, as 100% growth rates are extremely rare in most games. (Seisen is a bit of an exception here, because of the way stat inheritance and holy blood works -- keep reading!)
The generational system in Seisen makes for some characters who are overpowered compared to most FE games; the second gen kids get the base stat growths of their same-gender parent, plus half the growths of their other parent, and then possibly bonuses for having holy blood on top of that. So there are quite a few characters who can go over 100% in certain areas -- usually HP, which tends to have the highest growth rates, but it's possible in other stats if you shoot for the right pairings.
Fun trivia here, however, is that Leaf's family is one of the two in the game whose inheritance is backwards -- Leaf gets his base growths and inherits his equipment from his mother, not his father.
AVERAGE STATS AT MAX LEVEL:
HP: 70
Strength: 27 (maxed)
Magic: 9
Skill: 27 (maxed)
Speed: 27 (maxed)
Luck: 15
Defense: 27 (maxed)
Magic defense: 10
The HP cap in Seisen is set at 80, and natural stats can never top 30 -- though most of them will cap lower, depending on class; Leaf's magic and magic defense cap at 22, for example. Luck can always hit a max of 30 for every class, but very few classes allow 30 in another stat.
This was figured by taking his growth percentages and multiplying them by 29 levels of growth, then adding in the bonuses he gets from promoting classes. (Base stats here were assuming his parents were both leveled to their max, as that affects the stats he starts with.) This should of course be taken with a grain of salt, because as every FE player knows all too well, the RNG is a fickle mistress and it's quite possible to have good characters come out badly and poor characters turn out well. But here, it serves to pretty much drive home the point that Leaf is a physical powerhouse.
HOLY BLOOD:
Minor Baldo, minor Noba. This is mostly good for bonuses to stat growth; although Baldo confers an extra rank of sword proficiency and Noba confers an extra rank of lance proficiency, Leaf promotes to Master Knight and thus has no use for either of them because he already has A rank in both.
The bonuses are factored into his stat growths above, but in the interest of completion, they are as follows:
Minor Baldo - +20% HP, +10% strength, +10% skill, +10% luck
Minor Noba - +20% HP, +10% strength, +10% speed, +10% defense
WEAPON PROFICIENCY:
Sword: A
Lance: A
Axe: A
Bow: A
Fire magic: A
Thunder magic: A
Wind magic: A
Light magic: C
Dark magic: -
Staff: A
So pretty much everything except holy weapons, and a couple of light spells that you'd probably want to give to a dedicated mage anyway (you don't get any dark mages in your party in Seisen). Not that Master Knights are overpowered or anything. :Db
SKILLS:
Continue - extends combat for another round, with the chance of activating being (attack speed + 20)%.
Critical - allows critical attacks, at the rate of (skill + weapon kill bonus)%. Yup, back in ye olde days of Fire Emblem, you could only crit if you had the skill or if you managed to fulfill one of a couple other special conditions.
Pursuit - allows double attacks if the unit is sufficiently faster than the enemy being attacked. Yeah, you used to need a skill for that, too.
He doesn't start with the Pursuit skill, but as I was reminded after posting this...it's a class skill for Master Knights. lol overpowered. :'D
EQUIPMENT:
Light Sword - Inherited from his mama. In close quarters, it's just your average sword; attack power falls between the steel and silver weapons, so it's respectable, but certainly nothing on the level of the holy weapons. However, when used at range, it casts Lightning (which can be misleading if you're not familiar with FE, but that's a light spell, not a thunder spell). In Thracia 776, it also had a third function, where it could be used as an item to restore Leaf's HP. So it's a pretty sweet weapon overall, as well as being his only memento of his mommy.
Magic Ring - I like to give him one so he can get better mileage out of that sword (and so I can give him an elemental spell for the hell of it). Grants +5 magic. :|b
Tornado - The A rank wind spell. Nothing much special to add here.
Steel Lance - B rank lance, since I usually save the better ones for dedicated lancers.
Silver Bow - A rank bow. I have more awesome bows lying around than people to use them, so he can have a good one.
...no axe because axes are kind of the short bus in most situations. woooooo.