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!"-Quote from Chapter 13.

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 and, thanks to Ms. Smith not wanting to bother, are still thought of that way outside of the people she interacts with.

Appearance
She is a large mass of blue liquid slime and is thus transparent and gelatinous, she also has a mass of green "hair" with three yellow "antenna" on the tips. She can shape herself into a somewhat humanoid form that, according to Miia and Cera, is very rare 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.

In Chapter 9, she was given a rain-jacket to wear since anything else she wore instantly got wet.

Personality
Suu's method of learning is through mimicking the actions of others; such as Miia, Cera, and at times, Papi. She can be somewhat absent-minded at times; such not knowing that she almost drowned Kurusu and Miia by hugging them, and accidentally adsorbing many objects around the house. She also seems to be very child-like, as shown when she was playing video games with Papi.

However, it's implied that Suu may be not as simple minded and childish as she pretends to be, as seen when she cured Kimihito of his illness by herself and talked without mimicking and when she ruined the directors plans by reading his thoughts and revealing them to everybody.

Suu appeared quite impressed by the protective concern Papi has shown towards her, and seems to have consciously chosen her friend as a role model.

Skills and Traits
Since Suu is made entirely of a liquid-like substance, she is immune to most physical damage, she has the ability to change/manipulate her size and cling to the ceiling. When she absorbs more water she can shoot it out from certain parts of her body like a hose. Suu also seems to be a fast learner, having learned to talk 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.

Much like Dopple, Suu can mimic the appearance of anyone, however unlike her there is one drawback-her antenna always sticks out of the top of her head. She can also alter her body to mimic the appendages of her friends, such as Papi's wing's or Miia's tail. 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. Suu is also capable of mimicking people's voices perfectly.

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.

When touching someone's head with her antenna, she can view their memories, thoughts and emotions.

Suu becomes "diluted" when she is submerged in a large body of water, and it takes time for her to reform and drag herself out of it. However if she falls into a huge enough body of water like a river, the dilution effect would be serious enough to kill her. However, drying her out shrinks her and makes her lose her humanoid form, making it somewhat akin to starving her. To the point where she will practically molest anyone with even a trace of moisture on them.

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.