Belgium is a supporting female character representing the Kingdom of Belgium. Her appearance is distinctive, typically featuring wavy blonde hair that has been depicted with varying shades, from very light blonde to a darker brown, depending on the illustration or season of the anime. She has light green eyes and is often drawn with a unique cat-like mouth. A constant element of her design is the ribbon she wears as a headband, which is commonly shown in green, red, or black. She is considered a tall character, and her attire varies, including a khaki military uniform similar to those worn by other European nations, as well as a more formal aqua dress with a lavender bolero for world meetings.

In terms of personality, Belgium is described as mature, cheerful, and diligent. She presents herself as a kind and friendly older sister figure, but with a meddlesome or nosy side. Despite having features that stand out, she prefers simple, inconspicuous clothing and does not like to be the center of attention, a trait she attributes to being surrounded by more flamboyant nations. However, this gentle demeanor belies a fiercer nature; official profiles note that she is "scary when she's mad." Her speech patterns include a Shiga dialect, and she is characterized as a tsukkomi, or "straight man," a role that involves playfully correcting or reacting to the absurdity of those around her.

Belgium's motivations and role in the story are often tied to her relationships with her family and her desire for a peaceful, prosperous life. She is the younger sister of the Netherlands and the older sister of Luxembourg. Her relationship with her brother Netherlands is particularly complex and a key part of her background. While they have difficulty getting along due to historical conflicts—such as when Netherlands fought for independence from Spain while she remained—she still finds him cool and enjoys spending time with him, even if she thinks he is overly stingy. As a nation that has historically been a crossroads and a battleground in Europe, she is independent and does not enjoy being told what to do. Her primary motivations revolve around maintaining her high standard of living, enjoying gourmet food and tasty snacks, and looking after her younger brother, Luxembourg, for whom she worries.

Her role in the narrative is that of a supporting character who often appears in contexts related to European history and her family dynamics. One of her notable early speaking roles involves an interaction with a young South Italy, who had a crush on her. She is shown to be kind and patient with him, teasing him gently but also feeling bad when she upsets him. She also maintains a positive relationship with Spain, having stayed under his care when her brother left. Her kindness extends even to strangers; in one notable incident, she showed compassion to a naked Parallel Spain who had come to attack her, giving him her scarf because she was worried he was cold. This act highlights her fundamental nature as a caring individual, even in strange circumstances.

In terms of development and notable abilities, Belgium is known for her love of gourmet food, with a particular fondness for making waffles and chocolate. She is so particular about her cuisine that she is not a fan of foreign waffles. An interesting detail about her abilities is that she is a skilled businessperson, noted to have been a child prodigy in trade and even better at business than her brother when they were young. Her connection to her people grants her the typical abilities of a nation personification, such as enhanced strength and endurance beyond that of a normal human, as well as an immortality tied to the existence of her nation and its people. She has also shown a surprising tolerance for bizarre situations, such as nonchalantly encountering a naked intruder, indicating a level of resilience born from a long and often turbulent history.