Marv is a mysterious great existence responsible for the Sleepwalker incidents in Eastside, the subject of the Dreamy Bubble event. It feeds on human happiness by secreting a substance called Ambrosia, which when ingested causes individuals to fall into a supernatural sleep filled with joyful dreams. While they are in this state, they expel negative emotions in the form of dangerous spiritual contamination.
People afflicted by this condition are called Sleepwalkers, and it is said that this form of mind control power is similar to that possessed by Catalyst Sinners. In addition to vomiting black goo, Sleepwalkers also suffer from uncontrollable lethargy, intermittent memory loss, dispiritedness, and a sharp drop in metabolism when sleeping. One Sleepwalker reported sleeping for 57 hours straight, and wasn't dehydrated or hungry upon waking up.
Because the Chief doesn't dream, Ambrosia has no direct effect on them. Instead, by consuming Ambrosia, they can enter the same dream as another nearby person who is under its spell. This is notably similar to the ability possessed by Marv (and by Serpent while in possession of Marv's heart) to enter and travel between the dream realms of various Sleepwalkers.
Marv's origins are mysterious and unclear. It was originally believed that Serpent was the creator of Ambrosia and that Marv was created by her powers when a Sleepwalker transformed into a Corruptor, but in truth Serpent acquired Ambrosia after encountering Marv by chance outside the Endless Carnival. It is unknown what Marv's connection to the Carnival is, but the most likely explanation may be that it was an unusually powerful Corruptor created by the Black Ring, as its fixation on happiness as a source of energy is similar to that of the Ring's Corpus. Marv is also able to detect the presence of a Corpus inside the Chief, and like many other great existences, seeks to consume them. It wonders if, after devouring a Corpus, it would be "worthy" of creating its own Black Ring.
During Marv's encounter with Serpent, Marv gave her its heart as a concession to stop her from killing it. Its heart, known as the Bottle of Dreams, allowed Serpent to create Ambrosia independently and to control the dream worlds of Sleepwalkers.
"A few years" before the Dreamy Bubble event, Doyle had a case, one of a series, of someone with the codename Marv who was addicted to a drink that gave him happy dreams and let him escape from suffering. He was sapped dry and had a mental breakdown while sleeping, and turned into a monster. The man was then handed over to the FAC and deleted from the Public Security Bureau archives. It is not clear whether this is the Marv's true origin, as that would imply that Ambrosia pre-exists his transformation into a monster, or whether this was before or after Marv and its Ambrosia came under Serpent's control.