|WARNING: This article contains spoilers for the main story of OMORI.|
Marina is a light pink-skinned humanoid character with several cephalopod-like features. In place of arms she has two long tentacles, and she has two tendrils on the sides of each of her legs. Her hair is a hot pink and forms shapes resembling more tentacles. She has sharp, shark-like teeth and yellow eyes. All of her tentacles fade to yellow at their ends. Prior to her encounter in the main area of Humphrey, she wears a white lab coat with pink stains on it, presumably from the bodily fluids of her victims. In the latter half of this encounter, she discards the lab coat, revealing that under it she wears a fuchsia one-piece swimsuit with a cutout in the middle, exposing most of her belly, and a gradient that fades to yellow towards the top.
Marina is the most manic and unhinged of the Slime Girls, being short-tempered, resorting to violence quickly, and performing gruesome, Frankenstein-esque experiments in her sector of Humphrey. She apparently has repeatedly stolen from Molly's sector in the past. She speaks in a much less professional manner than her siblings, and is shown to be rather careless with her experiments.
Marina is the middle sister of the Slime Girls, and lives inside of Humphrey with her sisters. She has been performing experiments to create life since she was "only a baby", creating the NPCs Fuchsia, This Guy, Porinu, Applefly, Burro, Peeve, Sully, and Parsnip (in that order), as well as at least 658 other unknown experiments, an unnamed alleged 667th experiment, and the Sprout Mole?? enemies. She is apparently wanted outside of Humphrey for murder, according to a billboard on the Underwater Highway with her image on it.
Two Days Left
Omori and friends first find Marina at the end of her lab, where she is working on a suitor for Sweetheart. Once done, she reveals this suitor, who Sweetheart is disgusted by. She storms off, and Marina follows, demanding payment for her work.
After collecting the Slimy Key Card and using the Humphrey elevator, Omori's party finds Marina, Medusa, and Molly confronting Sweetheart, and Molly asks her for the 1 million clams she owes them. Sweetheart refuses, claiming that Omori and friends can pay for her before fleeing. Marina proposes they sell the children's parts, but Medusa insists they simply ask for payment from them, to which Aubrey refuses. Upon hearing this, the Slime Girls become aggressive, eventually leading to a boss fight.
Upon being defeated, the Slime Girls briefly argue over how they're going to feed Humphrey without clams, and are then eaten by him.
One Day Left
On the True Route, Marina is not seen again after Two Days Left. On the Hikikomori Route, Marina can be found inside of her lab again, now reduced to merely a head due to Humphrey digesting the rest of her body when he ate her at Two Days Left. Here, she will apologize to the party for trying to kill them, saying it was "just a spur of the moment kind of thing." She will then give the player the quest Marina's Operation, in which they must hunt down a Lab Rat from her sector to secure Rat Meat for her to create a new body with. Upon completion of her quest, Marina will reward the player with a Finger.