I've never run a campaign myself, only been a player, so can't offer you much from a DM perspective. But I got bored a little while ago and came up with class builds for a bunch of KH characters. Maybe you can find some inspiration from these? Obviously they can be flavored/homebrewed.
Sora
- Paladin, Oath of Devotion
- Multiclass: Wizard, School of Evocation
Riku
- Paladin, Oath of Redemption
- Multiclass: Sorcerer, Shadow Magic
Kairi
- Cleric, Peace Domain
- Multiclass: Paladin, Oath of Devotion
Terra
- Fighter, Rune Knight
- Multiclass: Sorcerer, Stone Sorcery
- Multiclass: Barbarian, Path of Wild Magic
Aqua
- Wizard, Bladesinger
- Multiclass: Paladin, Oath of the Watchers
Ven
- Rogue: Swashbuckler
- Multiclass: Sorcerer, Storm Sorcery
- Multiclass: Cleric, Light Domain
Roxas
- Fighter, Samurai
- Multiclass: Cleric, Twilight Domain
- Multiclass: Paladin, Oath of Vengeance
Xion
- Barbarian, Path of the Beast
- Multiclass: Cleric, Twilight Domain
Namine
- Wizard, School of Enchantment
Mickey
- Cleric, Light Domain
- Multiclass: Paladin, Path of Devotion
Donald
- Wizard, School of War Magic
Goofy
- Paladin, Oath of the Crown
Xemnas
- Monk, Way of the Astral Self
- Multiclass: Wizard, School of Abjuration
- Multiclass: Artificer, Artillerist
Xigbar
- Fighter, Renegade
- Multiclass: Ranger, Horizon Walker Conclave
- Multiclass: Wizard, School of Graviturgy
Xaldin
- Fighter: Cavalier [Dragoon]
- Multiclass: Sorcerer, Storm Sorcery
Vexen
- Artificer, Armorer
- Multiclass: Sorcerer, Cryomancy
Lexaeus
- Fighter, Champion
- Multiclass: Sorcerer, Stone Sorcery
Zexion
- Wizard, School of Illusion
- Multiclass: Sorcerer, Aberrant Mind
Saix
- Barbarian, Path of the Berserker
- Multiclass: Druid, Circle of the Moon
Axel
- Sorcerer, Pyromancy
- Multiclass: Rogue, Assassin
Demyx
- Bard, College of Creation
- Multiclass: Sorcerer, Sea Sorcery
Luxord
- Rogue, Wild Card
- Multiclass: Wizard, School of Chronurgy
Marluxia
- Druid, Circle of Twilight
- Multiclass: Warlock, Hexblade
Larxene
- Monk, Way of the Sun Soul
- Multiclass: Rogue, Swashbuckler
Master Xehanort
- Warlock, The Seeker
- Multiclass: Sorcerer, Clockwork Soul Sorcery
Young Xehanort
- Paladin, Oathbreaker
- Multiclass: Wizard, School of Chronurgy
Ansem SoD
- Sorcerer, Shadow Magic
- Multiclass: Warlock, The Fiend
Terra-Xehanort
- Sorcerer, Shadow Magic
- Multiclass: Barbarian, Path of Wild Magic
- Multiclass: Warlock, The Fiend
Vanitas
- Warlock, Hexblade
- Multiclass: Sorcerer, Shadow Magic
Riku Replica/Dark Riku
- Fighter, Eldritch Knight
- Multiclass: Sorcerer, Shadow Magic
Eraqus
- Paladin: Oath of the Ancients
- Multiclass: Barbarian, Path of the Zealot
- Multiclass: Fighter, Samurai
Yen Sid
- Sorcerer, Divine Soul Sorcery
- Multiclass: Wizard, School of Lore Mastery
- Multiclass: Druid, Circle of Stars
Maleficent
- Druid, Circle of Wildfire
- Multiclass: Sorcerer, Draconic Bloodline
Ansem the Wise
- Artificer, Alchemist
- Multiclass: Cleric, Knowledge Domain
Master of Masters
- Wizard, School of Divination
- Multiclass: Cleric, Trickery Domain
Luxu
- Rogue, Mastermind
- Multiclass: Wizard, School of Conjuration
Ira
- Barbarian, Path of the Totem Warrior
- Multiclass: Wizard, Order of Scribes
Invi
- Barbarian, Path of the Totem Warrior
- Multiclass: Rogue, Inquisitive
Aced
- Barbarian, Path of the Totem Warrior
- Multiclass: Fighter, Banneret
Ava
- Barbarian, Path of the Totem Warrior
- Multiclass: Cleric, Peace Domain
Gula
- Barbarian, Path of the Totem Warrior
- Multiclass: Rogue, Scout
Ephemer
- Paladin, Oath of Devotion
- Multiclass: Druid, Circle of Dreams
Skuld
- Paladin, Oath of Devotion
- Multiclass: ???
Brain
- Fighter, Samurai
- Multiclass: Wizard, Technomancy