Your Tamagotchi will take the costume off at bedtime. If I recall correctly, for V3 Tamagotchis, it will still be available in your item list afterwards.
it will come off when the tama goes to sleep. and yes u will still have it in your items, unless u trash it. (to trash something, go to the PRESANTS list and when you see the item you want and it says WRAP? press a and it will show a trash icon. press B and the item will dissapear. hope this helps