hmmm the only gacha Im playing is genshin so I'll just focus on the playstyle/gimmick

Kingston ✦✦✦✦✦: Main DPS // Sword // Electro
For the E skill: Sth like Candace/beidou sheild/counter but... like the fire emblem characters in smash bros bc its so fun to time a counter correctly hehe
For the Ult: sth like Hu Tao's/Xiao where their hp drains when they use their ults bc the gimmick would be Than... possession but it gives some stat boost and makes him stronger while draining his HP while attacking... so trade off...

And maybe a special dash like mona/ayaka but he turns into a lil shadow and zoom...

Ruth ✦✦✦✦✦: Support or Sub DPS // Bow // Cryo
For the E skill: some kind of ability that pulls nearby enemies toward her like sucrose/kazuha e and applies cryo
For the Ult: Kinda like Xing qiu's ult where she summons ice arrows and they attack in unison with the on-field attacker but it's a rebound shot... so her arrows will ricochet off enemies as long as there's more than one and deal extra damage the more enemies there are around

I can't think of playstyles for the other two x'D maybe I'll come up w sth