Excel is the title character and the primary agent of the organization known as ACROSS. Her true name, age, and place of origin are generally unknown, with "Excel" being her code name; in the manga, she occasionally adopts the pseudonym Hanako Dosukoi. Her background is deliberately mysterious, though it is suggested she joined ACROSS not long after graduating from high school.

In terms of personality, Excel is defined by her overwhelming and often misplaced enthusiasm. She approaches her work with an excess of determination and energy, but this is almost always undermined by a profound lack of foresight and common sense. She has a tendency to speak extremely rapidly and often refers to herself in the third person, making her difficult to understand. Her cheerfulness is boundless, and she is prone to long, manic monologues and conversations with herself, especially when alone. She can be seen as both impossibly lucky and unthinkably unlucky, often surviving catastrophic events through sheer resilience only to fail at the simplest objectives.

Excel's primary motivation is her fanatical, schoolgirl-like obsession with her master, Lord Il Palazzo, the leader of ACROSS. Her devotion borders on religious fervor, and she dedicates every action to his goal of conquering the corrupt world. She frequently speaks of saving her purity for him and seeks his approval above all else, though he often displays little patience for her antics. Her role in the story is as the chaotic engine of ACROSS's operations, sent on missions to destabilize the city of Fukuoka. However, she rarely, if ever, completes a mission successfully through her own planning; any successes are usually attained by happenstance or with the help of others.

Her key relationships define much of her daily life. She shares a complex dynamic with her colleague Hyatt, a frail woman who constantly coughs up blood and dies spontaneously. Excel treats Hyatt with a senpai-like affection, calling her "Ha-chan," though she is often jealous that Il Palazzo favors the calmer Hyatt. In the manga, she also clashes frequently with Elgala, a rival agent with refined tastes whom Excel views as a snob and a threat to her position. Outside of ACROSS, her neighbor Watanabe works for the city's security force, creating an adversarial relationship. She also keeps a small dog named Menchi, whom she explicitly designates as her "emergency food supply".

The character undergoes notable development, particularly in the manga. For a significant period, Excel suffers from memory loss and lives a peaceful life with the Shiouji family under the assumed name Teriha Shiouji. During this time, a robot duplicate (Ropponmatsu) takes her place in ACROSS, acting as a cool and efficient version of herself. Upon regaining her identity, Excel grapples with the memories of that life, revealing hidden depths and a longing for normal friendship that complicates her fanatical devotion to ACROSS. Later, her body is heavily modified, transforming her into an indestructible robot, and she eventually awakens as a being known as a "Brownie," granting her reality-altering powers.

Despite her ineptitude, Excel possesses several notable abilities. She has superhuman durability, routinely surviving explosions, falls from great heights, and being impaled, often without lasting injury. She is highly skilled in survival tactics, including trap installation and hand-to-hand combat, and has demonstrated proficiency in nearly every sport. She also possesses a bizarre, minor ability to warp reality, such as changing the animation style of her world or using a martial arts technique that transmutes people into weaker creatures. Her defining ability, however, is her relentless, unstoppable energy, which allows her to keep fighting and moving forward long after any normal person would have been defeated.