Low-energy type. A 17-year-old girl with blue hair wearing a white and light blue sailor uniform. A realist who loves coffee and dislikes water and the sea. She desperately solves mysteries in a world of terror to protect her childhood friend Lulu. By touching a tank, she can turn into a jellyfish and enter it, but if she uses this power too much... she won't be able to return to human form. She is unaware that Retro is the one who saved her in the past. She calls Retro "Colonel."

Suzu's childhood friend, natural and easygoing. A slow-paced girl with purplish hair wearing a white and light pink sailor uniform. A free-spirited and gentle personality whose catchphrase is "maybe~." She is the reason Suzu begins her search. Like Suzu, she can turn into a sea slug and enter tanks by touching them (unconsciously). She calls Suzu "Su-chan."

Tsundere, easily swayed. A petite and cute girl in a large pink and white skirt. Having been trapped in the aquarium for a long time, she has built unique relationships with the other members. In truth, her younger brother (the bratty guy) is sick, and she was told by Chris, who holds the Deep Sea Stone, that he would be cured. As payment, she was trying to deceive Suzu and the others to hand them over to Chris. However, her brother is already cured and she was actually being tricked by Chris, though she doesn't know it. She can turn into a seahorse Creepy. Upon learning she was deceived, she becomes distrustful of humans, but she opens her heart to Lulu, who responds innocently no matter what Kitty says.

In reality, an albino Great White Shark who saved Suzu long ago. A tall woman carrying a sword and wearing an outfit resembling a military uniform. She is cool-headed, highly capable in combat, and caring. She actually saved Suzu from drowning in the past, but was attacked by humans who thought she was eating Suzu, leaving her body covered in scars. When she saved the young Suzu, she received a pocket watch embedded with a Deep Sea Stone from her. She stands against Chris to stop her but loses due to the difference in power, eventually teaming up with Suzu. She has been with Chris for a long time and knows everything about why Chris became the way she is. She is the one who created the world Suzu wandered into using the Deep Sea Stone. Breaking the stone in the pocket watch allows one to leave that world, but in exchange, Retro will... "disappear." Only Chris and Retro know this secret.

Scheming, gentle type. In reality, a nameless Clione. A woman claiming to be the Director's wife, wearing a white and pink maid outfit. She holds a major secret. She is the true mastermind... Chris once had an unrequited love for the Director, but the Director was supposed to marry a wife also named Chris. When he proposed, a person with a mysterious gun suddenly shot the Director dead. At that moment, she used the Deep Sea Stone to turn herself into a human and rushed to him. The Director said, "Chris... it's you, isn't it?" and she replied "Yes," and her love became twisted. She invites people into the world she created, puts them in tanks to turn them into Creepies, and pushes the dead Director (now just bones) in a wheelchair to show him the fish she has procured. In battle, her face and head split open, her cuteness vanishes, and she reveals a Clione's buccal cones.

Carefree and a bit ditzy. Formerly human (human name was Toy). A gentle Creepy whose face is hidden by a hood and whose lower body is that of a sea lion. He has no memories of his time as a human. He is easygoing, slightly airheaded, and acts mysteriously. He met Suzu and Lulu once at the real aquarium, but he no longer remembers. He has a Bianca Aquarium annual pass(?) in his pocket.

An intelligent hermit crab who shows Suzu the way through actions when she is in trouble, though he cannot speak. A hermit crab Suzu rescued along the way. He likes the cherry on top of melon sodas. He often sits on Suzu's head. Suzu refers to him as "you."