Frank Besson is a character who appears in the second season of the anime, introduced as the older brother of Muriel Besson and a significant figure from France. His background is firmly established in the world of organized crime, as he is identified as the head of a prominent French criminal family and a high-ranking member of the mafia. His connections extend internationally, as he frequently visits Japan to conduct business dealings with the yakuza.

Frank Besson possesses a personality defined by a cold and condescending demeanor, particularly towards those he considers beneath him. He holds a deep-seated disdain for Japanese people, looking down on them internally and referring to them with insults. This attitude is masked by a controlled exterior, but it surfaces in his interactions with Masamune Makabe and his friends. The most dominant and defining aspect of his personality is his extreme, overprotective love for his younger sister, Muriel. He is portrayed as a serious siscon who has established a personal moral code where making Muriel cry is the greatest sin, and disappointing her is the second greatest. He never forgives anyone who makes his sister cry and will take ruthless reprisals against them.

Frank's primary motivation in the story is the happiness and well-being of his sister, Muriel. When she becomes upset, he acts as an unstoppable force to rectify the situation, regardless of the rights or wrongs involved. His role in the narrative is that of an antagonist who creates a high-stakes conflict for the main characters during their class trip to France. After learning that Muriel's request for Masamune and his companions to serve as models for her manga was declined, leading her to cry, Frank becomes enraged. He confronts the group outside a museum, using his guards and implied threats to force them into spending their free time teaching Muriel about authentic Japanese culture and acting as her muses. This action sets the central problem for the arc, placing the protagonists in a coerced situation they cannot easily escape.

The character's most critical relationship is with his sister, Muriel Besson. His entire involvement in the plot is driven by his devotion to her, and he acts as an intimidating guardian who removes any obstacle from her path. His key relationship with the protagonist, Masamune Makabe, is adversarial and built on coercion. Frank holds a contemptuous view of Masamune and his friends, and he has no qualms about threatening them to achieve his goals. In a conversation with his sister, he is shown insulting Masamune and the others, referring to them with a derogatory term. He also gives Masamune the nickname "Chikuwa-kun".

Frank Besson does not undergo a significant personal transformation or development within the confines of the story. He is introduced as a formidable obstacle and largely serves as a catalyst for the events in the France arc. His power and authority remain unchallenged, and he achieves his goal of securing models for his sister, showing no change in his tactics or beliefs. Notable abilities are not tied to physical prowess or unique skills. Instead, his influence comes from his position as a powerful crime lord. His ability to command armed subordinates, threaten others with firearms, and impose his will through sheer force of authority and implied violence is his most notable trait. His intimidating presence and connections to the underworld are the tools he uses to manipulate the situation in his sister's favor.