Jellies are a type of Noise. Noise #29-#32 are in this family.
During battle, Jellies attack by sending lightning bolts to each other. If Neku gets hit, damage is dealt to him. Those who received lightning bolts soon after clone themselves, which may result in a Jelly Madchester/Swechno for a Madchester or a Jelly Neocoustic/Germanic for a Jelly Neocoustic. Jelly Neocoustics/Germanics also can attack by grabbing Neku by their big tentacle, then draining HP. Jellies can extend their arms, then drift around, causing damage and possibly immobilization.