Son Hak, also known as the Thunder Beast, is a central figure in the narrative, serving as the former general and chief of the Wind Tribe. Adopted as a grandson by the tribe's elder, Son Mundok, Hak grew up as a childhood friend to both Princess Yona and Su-won, forming a close bond with them from an early age. His past is marked by tragedy, as he was orphaned at the age of two when his parents died in a war.

Hak's outward personality is characterized by a mischievous and nonchalant demeanor, often expressed through teasing the princess. Despite this playful exterior, he is deeply loyal and professional, to the point of addressing his childhood friends by their formal statuses. His loyalty is not given lightly; it stems from witnessing the bravery of King Il, which inspired a sworn commitment to protect the royal family, a duty he initially found troublesome. He is fiercely self-sacrificing, showing a willingness to put both his physical well-being and his emotions on the line to ensure Yona's safety. This internal conflict is a key part of his character, as he consistently suppresses his romantic feelings for Yona, believing his role is to serve as a tool for her survival rather than a potential partner. This emotional distance can fracture when provoked, particularly by the Green Dragon Jae-Ha, causing Hak to act territorial and lose his composure.

Hak's primary motivation shifts from being the right-hand man to his best friend Su-won to becoming Yona's unwavering protector following Su-won's betrayal and the murder of King Il. Falsely accused of the king's murder and the kidnapping of Yona, he helps her escape the castle and ultimately abandons his prestigious position to join her on a perilous journey to find the four legendary dragon warriors. His role in the story is that of a formidable guardian whose skills are indispensable to the group's survival.

His key relationships are complex, especially with Su-won, whose betrayal deeply wounds Hak, as the two shared a strong bond and trained together in their youth. His relationship with Yona is the emotional core of his journey. While he initially keeps his distance, his true care and devotion slowly surface, showing a man willing to live and die for her. He also develops significant bonds with the other dragons, such as Kija and Jae-Ha, whom he comes to see as elder brothers, a dynamic that occasionally makes him feel vulnerable despite his strength.

Throughout the story, Hak undergoes notable development. While he begins as a warrior who follows orders, his dedication to Yona becomes a personal mission that defines his existence. He expresses that accompanying her and being overwhelmed by her strength and passion taught him the pride of serving someone. His initial reluctance to teach Yona how to use a bow stems from a fear that if she no longer needs a bodyguard, he would lose his reason to stay by her side.

In terms of abilities, Hak is renowned throughout the Kingdom of Kouka for his outstanding martial arts skills, which earned him the title Thunder Beast. He is considered one of the most formidable warriors in the kingdom, with a level of strength that even the dragon warriors, such as Kija, believe could rival their own supernatural powers. His proficiency with the Chinese guandao, a type of polearm, is unparalleled, and he demonstrates remarkable speed and physical strength capable of dispersing large clouds of fog with a single strike. Beyond his signature weapon, he is also a highly skilled swordsman and archer, showcasing a versatility in combat that makes him an exceptional threat on the battlefield.