Suu

"Well, it was Suu's fault that you caught a cold... I should apologize as well. But Suu doesn't mind! Because she loves her Master!" - Suu talking by herself for the first time.

Suu is a Slime Girl that snuck into Kurusu's house without notice while looking for sources of water. She is the  fourth girl to move in with him, albeit unofficially. Prior to her appearance Slimes were still considered fictional creatures by humans and, thanks to Ms. Smith not wanting to bother, are still thought of that way outside of the people she interacts with.

Appearance
As a Slime, Suu's true form is a large mass of a transparent blue gelatinous slime. She also has a mass of green "hair" with three yellow "antenna" on the tips. For easier interaction with Humans, Suu shapes herself into a somewhat humanoid form that, according to Miia and Cerea, is unheard of for a Slime to be able to do.

Suu normally shapes herself to be about the same the size and build as Papi, but when she absorbs a certain amount of water, she chooses a more mature form: she becomes taller, bouncier, and curvier than usual. The excess water goes straight to her bust and hips. When dehydrated enough, she becomes small enough to fit into a small bucket -- showing as only a head and hands peeping over the rim.

While usually naked, Suu occasionaly wears only a raincoat and boots, since any other clothes would just get wet and dirty from contact with her.

Personality
At first glance, Suu seems to be a childish and simple-minded individual. She has a rather innocent, happy and sunny disposition and seems to be pretty friendly. While she does not have a malicious personality, her apparent obliviousness to the world around her can cause her to sometimes endanger people's lives by accidentaly suffocating them with her own body.

Because of their similar personalities, Suu gets along very well with Papi, and they are frequently seen playing together. Suu appears to be quite impressed by the protective concern Papi has shown towards her, and seems to have chosen her friend as a role model.

Suu's main method of learning seems to be through mimicking the actions of others; such as Miia, Cerea, and Papi. It initially seemed like Suu was unable to talk, and the only things she said were phrases mimicked from others. Kimihito theorized that mimicking was how Suu communicated.

It's eventually shown that the childish, simple-mindedness and oblivious aspects of Suu's personality seem to be a façade she keeps up for yet unknown reasons. When she was taking care of a sick Kimihito, and the suggestions of the other girls were only making things worse, Suu dropped the façade and cured him herself. She then, in front of everybody, talked by herself to reveal she's fallen in love with him, to everyone's great shock. When called out on it, Suu immediatly returned to mimicking.

So far, both times Suu has dropped her charade was when Kimihito was either sick or in danger, suggesting that his safety is her greatest priority.

While normally friendly, Suu can be angered when succesfully provoked. So far, the only time this has happened was when both Papi and Kimhito, arguably the two people closest to her, were in danger. When Suu was angry, she showed willingness to resort to violence for the first time.

While her stupidity was already revealed to be a front, Suu is still a lot smarter than she seems. She correctly deduced the reason for Kii's rampage and figured out how to stop it.

Even before the revelation about her true personality, it was already strongly implied Suu was more intelligent than it seemed. She did seem to genuinly enjoy playing with the children. And went out of her way to save a girl's life, despite not mimicking it from anyone. She also very carefully chose her words to ask the girl if she was okay while still pretending to mimick. She even did the same thing in an attempt to assure Papi when it seemed she (Suu) might die. She even remained calm, despite freaking out earlier when Papi flew her over a lake. And when she was saved, she comfortingly assured Papi she was okay.

Skills and Traits

 * Invulnerability: Since Suu is made entirely of a liquid-like substance, she is seemingly immune to any form of physical damage, as seen when Centorea's sword just harmlessly passed through her. Even when she solidifies herself, she is still immune to any form of blunt impact, as seen when she used herself as a living shield to protect a girl from being hit by a truck.
 * Poison and disease immunity: Due to her unique biology, Suu cannot catch any Human diseases. She is also capable of consuming any foods that could result in food poisoning to both Humans and Extraspecies, without them having any effect on her.


 * Adhesive body: Suu also has the ability to stick to any surface, such as ceilings.


 * Water absorption: As it's seemingly her only basic need, Suu is capable of absorbing water into her body. All she has to do is make physical contact with water and it will be absorbed. However, it's not clear if it happens automatically or if Suu can control it. Depending on how much water she absorbs she can also change her size and body proportions. She can also expel her internal water in pressurised streams from any part of her body. Suu is also capable of filtering her own internal water.
 * Healing: Suu has shown that she is capable of curing at least a severe cold by having the sick person drink from her internal water. The exact process behind this hasn't been revealed.
 * Size shifting: While she was already shown capable of changing her own size to a limited extend depending on the amount of water she absorbed, the most extreme example was shown when she absorbed a tank full of experimental fertilizer. During that instance, Suu grew to gigantic proportions.

However, unlike Döppelgangers, Suu's shapeshifting has one major drawback: her antenna and signature eyes will always be visible. Also, repeatedly changing forms in a short time is exhausting for her.
 * Shapeshifting: Suu seems to have an almost complete control over her own biology, which grants her shapeshifting powers that might even surpass that of a Döppelganger like Doppel. For one, Suu can mimic the appearance of anyone, Human and Extraspecies alike. Suu's shapeshifting is so versatile, she can even transform into clothes or objects, like boxes. She seems to have enough control over her own anatomy that she can control her own solidity and texure, as seen when she transformed into clothes without making whoever wore her wet. Miia once thought of using this ability to have Suu be "worn" by Papi and Mero to disguise them as Humans. The disguises were capable of fooling Kimihito and Ms Smith.


 * Voice-Mimicry: Suu is also capable of perfectly mimicking people's voices, though she hasn't used it in combination with a disguise yet.


 * Mind reading: When touching someone's head with her antenna, Suu can view their memories, thoughts and emotions. Combined with her ability to mimic voices, it allows her to broadcast a person's toughts to everyone in her area.


 * Fast learner: Suu also seems to be a fast learner, having learned to talk interact with others in few days, and later on is able to mimic her friend's word-for-word in order to help them "talk" with Kimihito while he was sick, and eventually started speaking on her own.


 * Eidetic memory: It's implied Suu may possess eidetic memory, as she was capable of perfectly memorizing and mimicking everything that was said and demonstrated to her when Kimihito was sick.


 * Acting: Suu seems to be a convincing enough actress that she can fool everybody around her into thinking she's not as smart than she actually is.

Also, exposing her to higher temperatures dries her out. If she dries out, she shrinks in size, to the point where she can fit in a small bucket, and makes her lose her humanoid form. However, doing so is the equivalent of starving her, and during that time Suu will mercilessly molest anything with even the slightest trace of moisture on them, no matter the circumstances.
 * Weaknessess: Despite all these strengths, Suu is not invincible. Throwing her in a body of water can temporarily immoblize and stun her, as she becomes diluted. If the body of water is large enough, like a lake, the dilution effect would be strong enough to kill her. Heavy rain is also dangerous to her, as the impact of all the water drops dissolves any part of her body exposed to it.

Plot
Suu first appeared in a pot while Kurusu was teaching Miia how to cook. The slime latched onto her face and knocked her out before darting into a crack. After Cera tells him what it was, Kurusu goes to wet a cloth for Miia's head; only for the slime to pop out of the small bucket he was using. Cera tried to strike at it with her sword; with no success. Suu then rushed them and left; leaving them both covered in her slime.

The slime later watched Cera and Kurusu as they washed each-other in the shower, and later snuck into the shower. Both Cera and Kurusu panicked, and Cera wound up getting knocked out. The slime then wrapped up Kurusu and revealed a humanoid form; much to his shock. She then began to hug and wash him, mimicking the actions of the other girls while using her body to rinse him.

But then she accidentally absorbed his head into her body, almost drowning him. However Kurusu dove into the water to get her off by diluting her. Later, after she expelled excess water to reform back into her normal form, Kurusu, Miia, and Cera debated on what to do with her while she played video games with Papi in the background.

Trivia

 * She is the only girl not to be brought to Kurusu by Ms. Smith.
 * She is the only character who did not have a name, but rather one was given to her by Papi.
 * Like the other girls however, her name derived from her species name: Suu from Slime. Papi stated this was her reason for naming her so.
 * When Cerea mentions she's disappointed to learn slimes are not as easy to kill as RPGs had led her to believe, she is referencing the Dragon Quest games, where Slimes (the same color as Suu) are usually the very first random enemy encounter and are the easiest monster in the game series to defeat.
 * Suu has a tendency to spontaneously reveal new powers depending on the plot, a reference to the limited knowledge on slimes by the government as they were kept secret by the monster representatives.