If you lose several times in a row in the battle with Muffet, she offers you help in exchange for your soul (which so far doesn't change anything in the gameplay). If you accept, you get an item with a name similar to "Help from Chara"
You can get a cup of water from the water fountain next to Muffet's battle area. Just remove the clothes from the water fountain and fill the cup in the act; you can then use it in a battle via the item menu.
