Eclair Martinozzi is a fourteen-year-old girl who serves as the captain of the Praetorian Guard, the personal guard unit directly under Princess Millhiore of the Biscotti Republic. She hails from Biscotti and is the younger sister of Lorrain Martinozzi, the leader of the Biscotti Knights. Her name is derived from the French pastry éclair, in keeping with a naming convention found within the series. She is often affectionately called Eclai by those close to her.

Eclair has a short, bobbed hairstyle of green hair and blue eyes. She is a canine-eared person, and she possesses a subtle complex about her ears, which are smaller and droop down rather than standing up like those of many other characters. Her personality is defined by her serious, straightforward, and often stubborn nature. As a dedicated knight, she can be rigid in her thinking, which frequently leads her to become the chief source of exasperated commentary, or "tsukkomi," regarding the naive and unconventional behavior of the hero, Cinque Izumi.

She holds profound respect and deep loyalty for Princess Millhiore, to whom she has devoted her entire being as a knight. This devotion is the central pillar of her motivation and her role within the story. As the leader of the princess's guard, her primary duty is the protection of Millhiore and the defense of Biscotti. She serves as a guiding figure for Cinque, often tasked with teaching him the customs of Flonyard and fighting alongside him in various battles. Her role in the narrative is heavily intertwined with Cinque's, as she is a constant companion on his adventures and a key member of the Biscotti forces. She shares a close, long-standing friendship with her childhood companions, Yukikaze Panettone and Ricotta Elmar, forming a tight-knit trio.

Eclair undergoes notable personal development, particularly in her relationship with Cinque. Their first encounter is marked by a comedic misunderstanding where Cinque, unaware she is a girl, touches her chest. Initially, she is often annoyed by his foolishness and lack of common sense, leading to frequent quarrels. However, over time, she gradually develops romantic feelings for him. A significant turning point occurs when a magical mishap causes them to accidentally kiss, an event that constitutes a first kiss for both. Following this incident, Eclair struggles to正视 (face squarely) her own emotions and begins acting coldly towards Cinque to avoid confronting her newfound feelings. This internal conflict culminates in a crucial episode where, with the encouragement of her best friend Ricotta and a challenging confrontation with Leonmitchelli Galette de Rois, she learns to accept her feelings, though she continues to express them in her typically tsundere manner.

As a knight who has trained since childhood, Eclair is a highly proficient fighter, specializing in a two-sword style. Among the young knights of Biscotti, few can match her combat abilities. She wields a javelin as an alternative weapon and can utilize the supernatural energy of Flonyard, known as "Bright," to perform powerful techniques. Her personal Bright Equipment is the Bright Ice Sword (also known as the Light Rin Blade or Hikari Rin Ken), a large glowing blade she can summon and control with a floating arm guard. She is well-versed in Emblem Arts, powerful special moves. Her signature techniques include the Skycracking Cutter (Retsu Ichi Monji), a simple energy slash; the Skycracking Cross (Retsu Ju Monji), a more powerful cross-shaped energy blast unleashed with her twin swords; and the Flash Sky Light Fang (Retsu Senko Kiba), a potent blast of light energy. The princess herself has noted that Eclair is the most familiar with the use of the Emblem Cannon. She also develops a combination technique with Cinque called the Skycracking Double Cross (Retsu Daburu Ju Monji), where they simultaneously unleash their cross-shaped attacks.