Antinomia: FF6

(I had been wondering how PowerPanda had swapped Runic for Shock in the Divergent Paths hack, and I guess it must be using that)

Actually no. There's actually a fair bit of customization that went into it. Here's how:
1. Jump is now Celes's native ability (which, IMHO, fits her SO much better... I'll never be able to go back)
2. This allowed me to repurpose the DragoonBoots, so that instead of changing Fight->Jump, they change Runic->Shock. The relic was renamed "Shock Bangle".
3. All instances of the DragoonBoots in stores and in chests were replaced with something else. There's a massive treasure chest shuffle in the World of Ruin that shifts each character's upgrade item to a spot that is relevant to their story (all documented in a readme). The important part here is that the chest in the Ancient Castle that contained the "Rage Ring" now contains the "Shock Bangle".
5. GrayShadows coded some ASM for me that check's a sword's "Enable Runic" flag, and tied Shock to that, so that any sword that can't do Runic can't do Shock either.

For Leo's stats, the stats given to him for the single battle were WAY too powerful for him being a permanent character. I did a spreadsheet of all characters' stats to find the median number for each stat, and then calculated a fair but strong set for a physical character. He has the highest natural attack stat of any character, just narrowly beating Sabin, but his Magic pow. suffers for it. Here is what I ended up with, and you are welcome to just copy it if you don't want to do all of the math.

Code:

Vigor: 49
Speed: 29
Stamina: 36
Magic pow.: 24
Battle pow.: 30
Defense: 48
Mag. def.: 24
Evade %: 8
Mag. bl%: 10
Run success: 3
Extra HP: 47
Extra MP: 10
Lev. Factor: Add 2 on average (really only relevant for the 2 times you recruit him)

For equipment, since he has no "special" character-only equipment, I made his specialty being able to equip a wide variety of armors. So he can equip heavy armor (ie - Crystal Mail) AND light armor (ie - Gaia Gear), giving him some unique combo possibilities.

Okay, a couple of things regarding names and profiles.
NAMES
Names are stored on the character slot, and have little to do with the profile. In event code, you use the 3-byte code 7F XX YY, where XX is the character slot, and YY is the name assigned to the actor profile. This will assign any name you want to any character slot. This also applies to the name change screen. If you evoke the name change screen for character slot 0A, you will be able to change Leo's name at any time, so long as the 7F XX YY command is NEVER AGAIN applied to that character slot. For your hack, you said that you put Leo in slot $0A. Mog occupies that slot in the battle where you get 11 Moogles. If you shuffled Mog over to slot $0E even that far back, then the name will appear as "?????" until you use the 7F commmand.

In Divergent Paths, Leo is named the first time you see him in the military camp outside of Doma. I re-wrote the dialogue from Soldier A and Soldier B to talk about "our general" rather than "General Leo". He has the chat with the soldier from Maranda to establish his overall personality, and then you name him. As soon as you get out of the naming screen, the other soldier runs in from the left, calling him "General <name>" and telling him he's been recalled. All other characters were moved off of slot $0D for the rest of the game, and when you finally get him in the party, I finally assign his profile to slot $0D. One note learned the hard way: any time you are switching back to the main party, you need to change Actor $0A's profile to an empty profile. Otherwise, the guy on the airship who unequips the team will "rescue" Leo's equipment for the rest of the team.

EQUIPMENT
Equipment, in turn, is determined by the character's PROFILE, and NOT by their character slot. So if you want Leo to be equippable, you will need to actually copy his profile from $0F (original game) to $0A (your hack). If you don't want players messing around with his equipment, you have 2 options:
1. There is an event bit that will prevent the player from opening the main menu. It's set during the Maduin flashback, because Maduin doesn't have a portrait.
2. You can make a second profile for Leo in any of the unused slots $30-$3F (TORK through HO) that is his "Guest Slot", and then copy profile $0A over to character slot $0A when he joins. (This is actually how Final Fantasy Tactics covers guest characters prior to them joining your team. Some characters who drop in and out even have multiple guest profiles.)

In Divergent Paths, the way that I got around all of the equipping/shop graphics/etc. was by actually switching everything about Leo and Umaro: sprites, palettes, profiles, etc., then editing the sprites every time they showed up as NPCs. That allowed Leo to show up in the shops, and Umaro and Kappa are excluded (because all of their non-relic equipment comes from steals or chests).