Jun Arashiyama is a nineteen-year-old A-rank agent and the captain of Arashiyama Unit, affiliated with Border's Shinoda Division. He is a young man of tall stature with long, black hair and green eyes. As the leader of his squad, which is also known for its public relations duties, he is a familiar face within the organization.

Arashiyama has a generally carefree and playful personality, and he does not hold a strong animosity towards Neighbors. This open-mindedness is evident in his interactions with others, particularly his defense of Osamu Mikumo. When Osamu violated Border rules by using his Trigger to protect people from a Trion soldier attack, Arashiyama did not condemn him. Instead, he publicly thanked Osamu for saving his own younger brother and sister, Fuku and Saho, and stated that he would support him. This incident highlights his core motivation to prioritize civilian lives and his sense of gratitude over strict adherence to regulations. He is also notably protective and caring towards his siblings, to the point of being somewhat overbearing.

Within the story, Arashiyama plays a significant role as both a capable combatant and a moral compass. His unit is often part of major operations, including large-scale invasions. He has a close and trusting relationship with Yūichi Jin of the Tamakoma Branch, a bond officially labeled as "Trust" in Border records. During the Black Trigger Capture arc, Arashiyama chooses to assist Jin, repeatedly expressing faith in his actions even when questioned by other agents like Sōya Miwa, who are suspicious of Jin's motives. He also acts as a mentor and supporter to Osamu Mikumo's team, offering advice and encouragement. His capabilities as a leader and teacher are such that Kyōsuke Karasuma, a skilled agent from Tamakoma, considered Arashiyama a more qualified coach than himself to teach Osamu advanced shooting tactics.

Arashiyama holds the position of an All-Rounder, meaning he can fight effectively at both close and mid-range. He was formerly the ace of his own squad, but he later relinquished this role to his teammate Ai Kitora to better focus on controlling the battlefield and coordinating his unit. This decision demonstrates his strategic mindset and prioritization of team success over personal glory. He is considered an outstanding captain, praised for his ability to devise coordination-based strategies that best fit any given situation. Beyond his combat prowess, Arashiyama is a highly charismatic individual. His natural ability to communicate and win people over caught the attention of Border's public relations chief, Netsuki, which ultimately led to his squad being selected as the organization's PR unit.

In combat, Arashiyama utilizes a versatile Trigger set that includes multiple offensive and tactical options. For ranged attacks, he wields an assault rifle that fires Asteroid bullets, noted for their high rate of fire, and a lower barrel that launches powerful Meteor explosives. At close quarters, he employs the Scorpion blade and is known for using feints, creating openings for attack by tricking opponents into thinking he has other Triggers active. His skill set includes the Teleporter, an optional Trigger that allows him to warp a short distance, which he uses both defensively to dodge attacks and act as a decoy, as well as offensively to reposition behind an enemy for a surprise attack. His defensive abilities are complemented by Shield, which he often manifests in a unique rectangular shape. According to official parameters, Arashiyama boasts high stats in Defense, Command, and Special Tactics, reflecting his role as a resilient and strategic leader.