Umaru Doma

Umaru Doma (土間 埋), alternatively Umaru (うまる) in her indoor form, is the main protagonist  of the Himouto! Umaru-chan series. She has an unknown ability to switch between two modes: an "outdoor" and "indoor" mode, the latter dubbing her as a "himouto" by her older brother, Taihei Doma.

Outdoor Persona
Umaru's 'outdoor' mode often draws attention by those who pass by her due to her attractiveness. She has pale skin with a slender figure and a small face with rosy cheeks and round, dark brown eyes. Her long, honey-blonde hair reaches her waist with two shorter strands framing each side of her face and short bangs covering her forehead. Occasionally, she wears her hair into a thin ponytail.

At school, Umaru wears her school's typical uniform which consists of a long-sleeved, button-down white shirt tucked into a red pleated skirt, along with a thin, red ribbon around her neck and brown shoes. Unlike most other students, Umaru opts to wear thigh-high black stockings instead of the white knee socks. In the colder months, Umaru can also been seen wearing a maroon blazer with this uniform.

Outside of school, Umaru often wears fashionable, frilly clothes that most girls her age are seen wearing.

Indoor Persona
Her true form - her 'Indoor' mode - is the form of a toddler (and her voice and behavior are also like a toddler's), and is drawn in a chibi-like manner. This form is a representation of her childish, lazy, and messy, personality. She wears an orange hamster hoodie, a white long sleeve t-shirt, and berry-pink knee-length leggings while at home. This chibi form is sometimes referred to as her "hamster form".

UMR Persona
In the video arcade (as her "UMR" persona), she wears a black shirt and a crimson hoodie with brown khaki shorts and black thigh highs, with a crimson cap. She later wears a dark blue mask to hide her identity from Sylphynford.

In all of her forms, her face also has very rosy cheeks.

Personality
In her 'Outdoor' mode, she is described as the perfect teenage girl at school and around town. She is a student who is on top of her class (due to her outstanding grades), possesses an overall charismatic feel, kind-hearted, excels in everything she does, and despite her popularity, she remains humble, thus earning the admiration by her fellow classmates and faculty alike. She also portrays a very sweet and cute personality. Her voice is also calm, more feminine, and very refined.

In her 'Indoor' mode, she is the exact opposite; she reverts to her true, chibi form, lets out her secret otaku side, and barely contributes in doing household chores (even taking care of her own hamsters), all of which she leaves to her older brother, Taihei. She is very messy,and lazy, eats junk food, drinks very large amounts of cola, and is obsessed with stuffed cat dolls, playing video games, reading manga, and watching anime, she  usually refers to herself in the third person, She is also prone to whining/crying when things don't go her way, and behaves very strangely when not in public.

In the video arcade, she is a well-known and feared gamer who goes by the initials "UMR".

Trivia

 * "土間"(doma) is the dirt floor which is an entryway area of a popular Japanese house, while 埋まる(umaru) means 'buried' in English. Thus her name means 'buried in a dirt floor'
 * The word "himouto," used to describe Umaru, means "dried-fish little sister." It is a combination of "himono-onna", meaning "dried-fish woman" who's proper in public but lazy at home; and imouto, "little sister", but "himouto" can also mean "worthless lazy-bum sister".
 * For some reason, she is a little bit jealous of Kanau's flirting with Taihei.
 * Umaru highly resembles her mother as seen in Taihei's flashback in episode 7, but her mother is taller and has a more feminine voice.
 * She's not afraid to do anything that might cause her brother to be annoyed because she knows that he won't stay mad at her for long.
 * She also claims that she shows her love to her brother by whining and being/acting spoiled (as shown in the anime's opening theme).
 * In episode 2, Umaru's hair turned golden as a golden-yellow aura surrounded her when she saw her brother give her friend Nana Ebina the Necolumbus plush (Umaru made Taihei miss a day of work so he could help her win one), displaying a "Dragon Ball Z" parody as Umaru had turned Super Saiyan due to her selfish rage.
 * In the anime, while changing from her inside form to her outside form, she has the habit of saying  'Umaruun', which is just a dramatized extension of her name. That SFX is also shown in the manga. When she reverts to her inside form, the SFX shown will sometimes be "N-rumau/Nuuramu", which is actually a reverse-spelling of the above 'Umaruun'.
 * She has an intense fear of ghosts, even though she claims she does not.
 * She has two different methods of pleading with Taihei to get her whatever she desires: While in public, her "outside mode" has her politely pleading, or guilt-tripping/pretending to cry so people will hate Taihei and he'll be forced to do/buy stuff for her; while her "inside mode" uses actual crying like a baby (which she actually looks like), yelling and throwing extremely loud tantrums until her brother surrenders. She'll also try to throw a tantrum in this form while in public, if none of her friends are around.
 * Umaru is, as aforementioned, obsessed with cat plushies, because her brother gave her one from the game arcade 10 years ago.
 * The scenes where she "humps her brother's head", among others, have become popular GIFs among viewers and fans
 * Umaru also appears to have the traits of a rodent (specifically her hamsters).
 * The siblings used to live in a large apartment, but after Taihei moved to another one, Umaru moved in with him later on, apparently so she could attend high school.
 * She has a large amount of hamster hoodies in the drawer and in the closet.
 * It is said that it is because of the way Umaru treats her brother sometimes that proves just how much of a sister she is to him; also, they have the relationship they do because that's what's right for them as the siblings they are to each other.
 * Umaru can be seen drinking cola the same way a baby drinks milk from a bottle.
 * Besides being called a brat, people who've seen Umaru in action refer to her as a "hamster", a "hamster girl", a "midget".
 * In her chibi form, she has a tendency to roll around, whether she's throwing a tantrum or otherwise.
 * Umaru is very prone to crying (usually when Taihei refuses to buy her whatever she wants.)
 * When Umaru pouts, her face usually looks like a balloon.
 * Even though she is very childish around her brother, she acts even more so when she is referred to as Komaru when Kirie and Bomber are around.
 * Umaru also has a very strong imagination.
 * In chapter 30, when Umaru and Taihei go walking and go to a park, she becomes her chibi form and starts riding a bouncing animal, proving that she is still a child.
 * At one point in the manga, Umaru decided to abandon her himouto lifestyle because she believed that it wasn't good to keep secrets from everyone, but also believed that her friends would hate her if she revealed it. When she began to reveal her secret, however, her friends stopped her and told her that friends shouldn't share every secret they have with each other, and that they'd never hate her. When Taihei asked her if she was going to keep showing her different selves to her friends, Umaru says she shouldn't force herself. This scene was adopted in the 2nd OVA episode of the anime series.
 * Another theory is that the reason Umaru is so childish around her brother is because her mother seemingly passed away when they were young, and their father was too busy to raise her, and Taihei wasn't really in a spot to raise her either.
 * As aforementioned, Umaru's first name is written in two styles: Kanji (for her outside form) and Hiragana (for her indoor form). However, the word "umaru" is actually a combination of kanji and hiragana ("u" is written in kanji while "maru" is written in hiragana)