I didn't really have most of them planned out like that, but I think I can remember a few plans I had ready, the Gen 6 ones I do remember for sure!
Xerneas - pregnancy obsessed
Yveltal - always worried about hurting others thanks to her powers
Zygarde - Violently tries to keep balance
Diancie - Vain about her own beauty
Hoopa - Prankster hiding her real power
Volcanion - (had nothing for that one)
Missingno - I was going to add her to Kalos since there were no sub legends in gen 6, she was going to be in Lumios City as a reference to how saving outside in Lumios in X and Y could corrupt your save data before the game got patched. She was going to talk in computer syntax and error messages.
As for later generations, let me see what I can remember
Nebby (solgaleo and lunala) - No set personality other than thinking of the player as a childhood friend, since they evolved right in front off you. One would come from another dimension through an Ultra Wormhole and evolve the other way so you could get both.
Necrozma - Psychotic and murderous, claims to have killed Arceus in her own dimension, something the player's Arceus doesn't care about.
Magearna - Adores the player for repairing her broken body, acts like a typical maid.
Eternatus - Gentle despite her size, destroys things by accident
Calyrex - Regal and pompous, bosses around Glastrier and Spectrier, even though they mostly ignore her orders or use her as a fuck toy (I planned the horses to be futas)
Zarude - The player's Zarude was going to be the matriarch of her troop and would go with him so he'd keep their location a secret, she'd be like a strict mother.
The Loyal Three - Obnoxious frat bro personalities, always hyping themselves up and bragging about how great they are
Ogerpon - Sweet and adorable, but has an angry side that terrifies most other legends.
Anything else, I either didn't plan out, or I forgot what plan I had