|WARNING: This article contains spoilers for the main story of OMORI.|
From the outside, Humphrey appears as a cyan whale with small lumps growing on the right side of his face. From the inside, he instead appears to take on the form of sentient pink lumps. In both instances, Humphrey always is smiling a toothy grin.
While Humphrey appears to be goofy and fun-loving, as shown by the various antics the buds inside him get up to and his jovial attitude, in reality, he shows borderline sociopathic tendencies: He eats the Slime Girls without second thought the moment they are unable to feed him, and immediately turns on Omori and his friends, intending to eat and digest them when they fail to satisfy his hunger. Apart from showing sadism in his boss battle, as he mocks Omori and his team's efforts to survive against him, he also hints towards knowing the truth behind Headspace, noting how "ignorance is bliss" and that "this cycle will never end," both referring to Omori's manipulation and repeated resets of Headspace in an effort to hide the truth from Sunny.
The Branch Coral notes that age has "removed his conscience," and that he has "evolved into a parasite within himself," explaining his erratic and impulsive behavior.
After Omori breaks his friends out of their fraudulent contracts and frees them from Mr. Jawsum's employment, they find themselves increasingly listless as they slowly begin to forget about Basil's existence and their quest to find him. After encountering Sweetheart during their exploration of Deep Well, they follow her through Deeper Well and eventually come across Humphrey, who invites them inside his body to continue their pursuit after confirming that Sweetheart is also inside.
As Omori and his team explore, they come across the Slime Girls working in their individual laboratories on Sweetheart's commissions for a perfect replica of herself to marry. They witness her dissatisfaction with all three end results, Roboheart, Mutantheart, and Perfectheart, and her refusal to pay, before collecting a Slimy Key Card that grants them access to the deepest part of Humphrey - the Slime Girl's lair.
There, they witness Sweetheart being cornered by the three outraged sisters, who demand payment for their work. Sweetheart instead tells them that Omori's party can cover her one million Clam fees, before attempting to leave. Humphrey intercepts and eats her, noting that she didn't have a single clam on her, before the Slime Girls turn their attention to Omori and his team, who either cannot afford to pay, or refuse to, depending on the amount of Clams Omori currently has. The Slime Girls attack Omori's party in response, intending to enslave them and sell their body parts.
After Omori and his friends fight off the Slime Girls, they begin arguing about whose turn it is to feed Humphrey. He devours them as well, before turning his attention towards Omori's team, preparing to kill and eat them, prompting the boss battle against him.
After his eventual defeat, Omori and his friends are shot out of his blowhole. Humphrey complains about them giving him indigestion, as he sinks into the water. He is not seen again on the Sunny route.
If Deeper Well is revisited on One Day Left, Humphrey will have resurfaced, although he will still be hungry. If Omori feeds him the Giant Check that Hero receives from Mr. Jawsum, he will allow them to go back inside him to resume exploration. This grants access to the Boss Rush, as well as three new side quests from the Slime Girls, who request Omori's help in rebuilding their bodies after Humphrey ate them. This also allows for Mutantheart and Perfectheart to be fought.
See Humphrey (enemy).
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