Yeah basically you choose a single ball out of some options at the start of the game, and they have scaling which scales each time it uses the FIGHT button (usually), on that specific party member (cus grimoire), but your party is normal and not changed at all while in the overworld, but in battles you instead have just your chosen ball as the party member, also the attack stat only matters if you choose lance or grimoire (shield not being able to deal damage through the fight button at all due to having 0 attack)
Note: scaling carry's over through fights, all the balls have 100 max hp (cant go up), they all can act (or use spells if they have a spell), and they cant equip armor because their not in your party in the overworld
Sword: starts dealing 1 damage per attack, but the damage per attack goes up each hit, thats very simple
Shield: shield's attack stat actually matters, because its attack stat is always set to 0, meaning all the fight button actually does is give tp (or mercy when fighting gerson), and during enemy attacks your soul is always the green soul, but you can still MOVE the soul, meaning that your just the red soul but your green and have the shield (8 ways), also any time you block an attack, the damage it wouldve done to you if it hit you goes to the ENEMY which has the attack instead. oh btw jsyk ALL attacks would be blockable
Scepter: its basically sword but it also has heal prayer, also yes the heal prayer works the same way as attacking for this, meaning every second (YES SECOND THIS TIME) time you fight or use heal prayer, your damage AND hp given from heal prayer goes up by 1
Lance: this one doesnt scale damage from its FIGHT button, it actually has the same attack as (base) RALSEI, lance also has rude buster, but just like scepters heal prayer, the damage dealt by rude buster goes up by 2 (TWO THIS TIME) every time it fights or uses rude buster, during attacks there is a 25% transparent white line spinning around your soul, and every 4-7 seconds you stop controlling your soul and it starts moving in the direction that the transparent line is facing until it hits a wall (which is called jousting) and after you hit the wall you can control the soul again, while the soul is jousting you are IMMUNE to ALL damage
Grimoire: grimoire is like shield where it doesnt deal any damage through the fight button but it doesnt deal damage during attacks. grimoire has a spell (FULLY CUSTOM) where you target an enemy and it gets cloned to the party for the rest of THE CURRENT ENCOUNTER ONLY (same attack and defence) but its starting hp (max hp) starts at 1 (and scales by 1 every time you use the spell), if you already have 2 enemy minions in your party at the time then a random one of the two minions gets replaced
(this took so long to write...)