MARINA has a reckless and brash personality. She has a short temper which is shown when she yells at SWEETHEART for not paying her the clams she needed, or when she screams at KEL for biting her after they attempt to take him away to sell him for CLAMS. She also tends to be the one to angrily yell orders at HUMPHREY. She addresses SWEETHEART as "lady" in a somewhat rude manner and has a mischievous laugh, "kikiki."

Like her sisters, MARINA is a scientist, though she works with live test subjects and ever since she was a kid she has enjoyed taking apart creatures and putting them back together. Like a mad scientist, she has created at least 667 unnatural living experiments, having no care for her experiments' suffering. She appears completely desensitized to gore, screams of pain, and keeping small creatures in tiny cages. It's also stated that the HUMPHREY that work for her in her sector keep "disappearing", causing her to continually ask MEDUSA to give her more. She appears to be more fond of conducting experiments on her own time, showing low confidence in MUTANTHEART's existence after creating her and instead demanding the money.

Much to her sisters' annoyance, MARINA's horrific experiments are largely responsible for them being known as sea witches even though they consider themselves followers of science. According to MOLLY, MARINA likes "feisty ones" and she'd like to keep AUBREY, though it's not stated for what reason exactly. She acts in a more violent manner than her sisters, willingly combative, and immediately assumes that when SWEETHEART promises the children will pay for the costs, she means that they are to sell them for profit. Despite her compulsive behavior, she doesn't seem to hold long grudges.

After she loses her body and meets the children again, she says they shouldn't take her wanting to rip off their arms and legs so personally, as it was just a "spur of a moment kind of thing." She wishes to recreate her body out of quality rat meat and thinks that giving someone a severed finger is a good sign of gratitude, and needed to be reminded of the concept of giving things in return in the first place.