In Embodiment of Scarlet Devil, she is described as being quite strange and eccentric, which makes it difficult for others to understand her. She's also described as being "normally docile" and generally pleasant to be around, but somewhat of a loose cannon if left unwatched. In Bohemian Archive in Japanese Red, it's suggested that she isn't very good at talking to other people, though she can get rather sassy and uppity when talking with her sister. Her friendship toward humans is very low. Until the time when she met either Marisa Kirisame or Reimu Hakurei in Embodiment of Scarlet Devil, she claimed to not have seen any humans other than when they were prepared for a meal, although it's left vague as to whether that was her idea of a dark joke.

In Foul Detective Satori, her attitude was particularly impolite and informal, speaking in a very casual tone and generally doing as she wished without consideration of consequences, although she stopped short of seriously injuring anyone.

As a vampire, Flandre possesses superhuman capabilities just like Remilia Scarlet, such as enhanced physical strength, with the destructive power of oni and the nimbleness of tengu, as well as a vampire's bodily and regenerative ability. She can swing an oversized sword, though not as quickly as she would like to. Since she doesn't know the proper way to attack humans, she is unable to go easy on them and blows them away without a trace.

Destroying absolutely anything and everything Flandre can destroy anything she wants, which is truly a frightening ability with no real weaknesses. By moving the "eye" of the object, where the tension is highest, and by attacking this eye, she is able to destroy the target. She is able to move the "eye" of anything into the middle of her hand, then she pours her power into it or in other words, she crushes the "eye" and unconditionally destroys the target with just a squeeze. She states that everything has an "eye" and that she has the "eyes" of everything in her right hand. She destroyed a meteor with a single hand and without touching it, where Flandre stated that she "went squeeze and it went boom".[4] When Sakuya was locked in the basement with Flandre, she destroyed the lock of a door simply by holding its handle, and without making any visible explosion.[5] Everything that is made for her to play with ends up broken, which may mean why she doesn't have many playmates, to which it's recommended to never meet with her.

Flandre is able to clone herself as seen in one of her spell cards where she creates 3 clones that join her in the battle. The clones attack as they like which could mean they're able to think independently to some grade. It's in the category "Slave Type" so they're under the command of the original Flandre.

In her hand, she possesses an object that looks like a crooked clock needle (sometimes thought to be a Lævateinn) – this is a prominent peculiarity associated with Flandre, in which the name is derived from her second spell card Taboo "Lævateinn".

Flandre has lived for at least 495 years, but she has allegedly never left the grounds of the Scarlet Devil Mansion, and rarely leaves even the building itself, if ever. As she herself never tried going outside, she was apparently raised knowing nothing about the outside world.

In Embodiment of Scarlet Devil's official art, Flandre has red eyes and short blond hair, which is tied into a single ponytail on her left side. She wears a red dress and vest, with a pink shirt underneath and socks, and a pink mob cap with a red ribbon attached. She carries a matching wand with her, which might be the mythical Lævateinn. Arranged from her back to the tip of her wings her 8 crystals which are presented in seven colours: light blue, blue, purple, pink, orange, yellow, light green and finally light blue again. In Sunken Fossil World, the arrangement shifts by one color, presenting her wings in the following order: blue, purple, pink, light pink, yellow, green, light blue and once again blue, and there is more variation of hue within the crystals.

The wings are an irregular shape unlike those of any living creature, youkai or ghost, or a vampire's typical bat-like wings (or even organic matter). In an interview, Reimu says, "I doubt she can fly with those wings," and Hieda no Akyuu notes that there are plenty of those in Gensokyo who can fly without wings. In Whispered Oracle of Hakurei Shrine, ZUN says Flandre's wings originally were bat-like like her sister's, but due to them being countlessly destroyed and regenerated, they ended up in their current form, which was easier to regenerate.

Remilia Scarlet is Flandre's older sister. Flandre is stated to respect her sister and Remilia cares for Flandre as shown where she is considering getting her a good tutor in Imperishable Night, but it is unknown how often the two interact. Perfect Memento in Strict Sense states that it's rare (but not nonexistent) to see Flandre and Remilia together. However, they are seen depicted together many times such as in Forbidden Scrollery chapter 7 when they were looking at their pet Tupai and in Bohemian Archive in Japanese Red, which also shows that Remilia will at least allow Aya Shameimaru to interview Flandre in the first place. They also show up together in The Grimoire of Usami, Violet Detector, and Foul Detective Satori.

Although Flandre might respect her older sister, it's to the extent you'd expect a snarky child to. She is willing to outright state when she thinks Remilia is just spouting nonsense, at which point Remilia comes in with a comment that she shouldn't be talking about her older sister like that. Her treatment of her older sister became harsh and she even called Remilia "that person" (あいつ aitsu)

Not much is known about Flandre's relationships with other residents as she spends most of her time in the basement. Patchouli Knowledge is aware of Flandre's existence and the need to keep her confined to the mansion, as Remilia noted Patchouli was probably causing a rainstorm around the mansion to keep Flandre from getting out. Sakuya Izayoi also serves Flandre, as Flandre is the sister of Remilia, who is the head of the house. Thus chores such as preparing Flandre's food are also done by Sakuya.