*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".* “I’ll get serious next year.” 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. 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. 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.