They are red and maroon in color and resemble jellyfish or weeping flower buds. Jellyfish have no humanoid characteristics other than the two squat legs visible from below the petal-like frill.
When interacted with, the frill shuffles from side to side and produces a series of bell chimes. This triggers paraphysical events such as teleportation or twisted dimensions.
On the credits screen at the end of the game, they can be made to chime with the number keys. The lower Jellyfish reacts to the (1) key and makes a low sound, and the upper Jellyfish reacts to the (3) key and makes a higher-pitched sound.
Jellyfish are strange characters that provoke a lot of questions. Their lack of expression and strange behaviour make them very mysterious characters.
Most notably, despite being surreal creatures exclusive to Madotsuki's dreams, they appear in the real world after she jumps from her balcony. This may indicate that what was apparently the 'real world' was nothing more than another layer of Madotsuki's dreams, or may suggest that they were supernatural entities that always existed outside of Madotsuki's dreams and guided the events of the game from the beginning. The fact that they appear in the ending as well as next to Shitai-san is usually brought up as evidence that they are reapers of some kind, and are thus sometimes called "death gods" (死神, "Shinigami") in the fanbase. Alternatively, their appearance may just be symbol that Madotsuki will never escape her dreams, even in death.
They are also the only NPC character (other than Uboa and the Toriningen) to be immune to the knife effect.
This implies that they represent a very powerful force in Madotsuki's psyche on par with these other major characters, yet unlike Uboa and the Toriningen, Jellyfish show no obvious signs of hostility. This trait may also tie in to the theory of them being supernatural or deific figures. Further evidence to support this is the sound they make when interacted with, a chiming bell. In the Japanese Shinto religion, bells are often placed on shrines and are part of the ceremonial ritual associated with praying to the Gods.