Tatsumi
- Voiced by: Nobunaga Shimazaki
A high school student living alone in his deceased grandfather's house, until he rescued a freeloading merman and let him live in his tub. Very concerned about finances. Seems sullen and disinterested, but cares about his new friends a lot.
- Cloud Cuckoo Landers Minder: The one who's become responsible for Wakasa, and occasionally the other sea creatures. Usually he will entertain their whims and teach them about the human things they don't understand, but sometimes he's the one who has to reel them back in and snip their (usually Wakasa's) dreams before things get too out of hand.
- Minor Living Alone: He's still in high school and lives in his grandpa's house instead of with his parents and sister.
- Weirdness Magnet: Ever since he found Wakasa by chance, his bathroom is constantly attracting more wandering sea creatures.
Wakasa
The titular merman. A freeloader in every sense of the word, but with good intentions and a good heart.
- The Ditz: Rather clueless about most things, but that rarely diminishes his excitement about trying new things.
- Long-Haired Pretty Boy: He has long blond hair and is quite handsome.
- More than Meets the Eye: In Chapter 97, Wakasa reveals a more scaly and finned form, rising up out of the bathtub and controlling water in order to attack the scientist who tried to forcibly take him from Tatsumi.
- No Sense of Personal Space: Is very huggy and doesn't mind pulling Tatsumi into the tub to bathe with him, or hugging onto him whenever he's sitting too close to the tub for too long.
- Older Than They Look: He's at least 200 years old, if Chapter 91 is anything to go by, and is the second oldest of his friends, with Agari being the only one older than him.
- Trademark Favorite Food: He enjoys eating hamburgers. When Tatsumi mentioned a possibility of switching from hamburger meat to tofu to save on expenses, he was devastated.
Takasu
A handy octopus who is an old friend of Wakasa's, he's a bit jealous of the merman's new lifestyle, and occasionally tries to prove that he'd be a more useful bath mate to keep- when he's not just hanging around and having fun with the others.
- Butt-Monkey: Was only even invited over so that he could give Tatsumi a massage as a gift from Wakasa, gets zapped by a microwave while trying to fix it, Tatsumi can't help thinking about takoyaki when he's around, and when everyone gets together to play a card game he's the one who keeps missing all the turns. He's also apparently been chomped on by Agari before! If there's bad luck to be doled out and he's there, you can bet he's going to be the one suffering.
- Mr Fix It: Having 8 arms makes him a very handy guy. He's good with tools and fixing electronics, managing to fix Wakasa's water proof tv with just a screwdriver (after he crushed it by sitting on it).
Mikuni
An air-headed jellyfish who loves to wander. Without water, he will shrink down into almost nothing.
- Cloud Cuckoo Lander: He's incredibly forgetful and a bit of an airhead (or water head in this case). Despite needing to be in water to survive, he loves to wander about on land. He also often forgets that he shocks people by touching them, and accidentally fries Tatsumi on a couple of occasions. Being a jellyfish, which don't have brains, this all makes sense.
- Moe: A literal moeblob- aside from his hands, face, and a bit of hair, the rest of his body is just a featureless blob of water. In most frames he's just drawn as an adorable blob with a beaming face, often shrinking.
- Shock and Awe: Being a jellyfish, it's more of a sting, but it acts about the same. When tiny, the shocks are more subtle and act almost like a massage, but when massive, it's enough to knock Tatsumi unconscious.
Maki
A depressive snail with low self esteem. He is prone to self deprecation.
- Self-Deprecation: Is constantly apologizing whenever he does something 'bold' that doesn't pay off, claiming that he's worthless. While at first Tatsumi seems to feel bad for him, at the end of their first encounter it seems he and Wakasa are both a bit annoyed by this habit.
Agari
A silent shark who is even older than Tatsumi. Though he looks intimidating, he's actually very shy, and speaks through animated gestures ("body language") rather than show his fearsome mouth.
- Elective Mute: Hates opening his mouth and is very shy around others, so he doesn't speak. Instead, he communicates with body language. Tatsumi and him bond easily through this.
- More Teeth than the Osmond Family: He's a shark, what do you expect? However, he's self conscious about it, and tries to avoid opening his enormous maw when he can.
Kasumi
Tatsumi's clingy little sister, she visits the house at least once a month. She is in love with her brother, and initially sees Wakasa as a rival. The only other human besides Tatsumi who knows about Wakasa.
- Big Brother Attraction: Kasumi has a lot of inappropriate thoughts about her older brother for someone her age. Has already claimed she will be his bride, tries to bribe Wakasa to take pictures of Tatsumi for her while she's not around, and then there's the butler chapter...
- Clingy Jealous Girl: Initially thinks that Wakasa is a woman that Tatsumi is hiding- but even after discovering he's a man, she still sees him as a rival for her brother's affection. Luckily, the two have a lot in common, so they get along more often than not.
Goromaru
A young starfish who starts hanging around Tatsumi's bathroom. He's a bit like Kasumi.
- Annoying Younger Sibling: Is clingy and show offy around Tatsumi, trying to get his attention, and glomming onto him as an older brother. Tatsumi is just as patient with him as he is with Kasumi, though.
- Love at First Sight: Immediately falls for Kasumi when he meets her.
Sousuke
Tatsumi's friend from school, a friendly guy who likes to joke around.
Echizen
A sadistic crab who is rather possessive of Wakasa. Used to be friends with Takasu, until they had a falling out.
- Childhood Friends: With Takasu.
- Sadist: Loves to watch Wakasa squirm and make him beg, complete with enjoying seeing him tied or chained up.
- We Used to Be Friends: With Takasu. They fell apart because they got jealous of each other hogging Wakasa's attention as children.
Makara
A young clownfish that Wakasa took under his wing out in the ocean. Their sex changes depending on the sex of the humans near them.
- Clingy Jealous Girl: Has shades of this when becoming female around Tatsumi, but after his gentle rejection, they backed off.
- Sex Shifter: Depending on the sex of nearby humans. When Tatsumi is around, Makara spontaneously develops breasts. When Kasumi is around, the breasts vanish.
- The Quiet One: Compared to the other characters, they're much more quiet and shy.
