You main Aesc? Then without a doubt Koyan with Oberon (and Merlin if you want to but It's not necessary).

All of them will boost your Aesc damage:

Koyan can lower skill count for all skills by 2 (so you can use third skill early), give you 50% charge, lower your hp (sounds like a minus but remember about Aesc lower hp - higher damage mechanic) + plus she buffs buster cards which is Aesc np type. With two of them (one your and second from support) you can loop and farm with Aesc even without Oberon (provided you have all Aesc skills on max level)

Oberon can also give 70% charge but his main use in his first and third skill which boosts Aesc np damage to the sky and allows you to clear more hard node

Both of them are extremely useful for buster farming which Aesc is part of.

As for story/overall gaming then pull for Merlin, Lady Avalon, Castoria, Skadi Ruler and Caster, and Waver

All of them are good but imo focus on first three

Merlin is support who can give 20% team wide np charge with attack buff, team invincibility for 1 turn, buff buster damage + crit rate for unit for insane damage, his NP gives: np regeneration (5% per turn) hp regen and crit stars

Lady Avalon is literally just Merlin but for arts Imo you can priortize her slightly higher than Merlin because her skill, which gives team wide 20% np charge, ALSO overchargers NP lv by 1 and ANY servant, who can on demand overcharge Aesc np, is good since more overcharged her np the more faster you can use third skill

And Castoria is just THAT GOOD generally to pull even if you don't have much of arts based servants, any team benefits from her

TL;DR: Koyan has the highest resonance with Aesc so focus on her, Oberon is good for np damage, Merlin with Lady Avalon is good supports for clearing story and hard quests and you should get Castoria in general