Seri is an NPC that changes costume positions: top, middle, and low.
Do note that costumes can be enchanted with costume enchants which correspond to the final position of the costume.
Example: If you change a costume from mid to low you can apply low enchants afterwards only.
She's located at upper 2nd room of main office (main_office 69, 309). It costs 20 GemCoin (50000) to change slot of the costume and only works if the costume is occupied on one slot only.