Pay for your inventory.

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I'm honestly curious... people who have more than 2 retainers, what are you even storing on them? I ask because I have multiple gear sets (every single available set for DRG minus a few pieces here and there plus a bunch of sets for other jobs), multiple weapons (Primal & Coil), dyes (all of the retainer venture ones), glamour gear, rare/expensive crafting mats, and various and sundry other things on mine and I still have plenty of space left.

Then you don't have as much as you think? I have one retainer entirely filled with weapons and gear for potential glamours, and one entirely filled with mats. It's not exactly difficult.