Mystery is portrayed as the most enigmatic member of the Saja Boys. He maintains a reserved persona, often striking bashful poses and remaining silent throughout his appearances as an idol. His concealed eyes make it difficult to read his emotions, leaving his mouth as the primary indicator of his expressions. Despite his withdrawn image, Mystery exhibits moments of unexpected aggression. Notably, he barks incoherently at a fan without provocation during a signing event. Additionally, after Jinu sacrifices himself and their plan starts to fall apart, Mystery is the first member of the Saja Boys to charge at HUNTR/X. Mystery first appears in the Demon Realm alongside Jinu during a meeting with Gwi-Ma, where Jinu proposes a new strategy to defeat the Demon Hunters. He proposes forming a rival K-pop boy band to undermine HUNTR/X, the current generation of hunters, by winning over fans’ devotion and weakening the Honmoon. This leads to the creation of the Saja Boys, with Mystery as one of its members. Mystery next appears in human disguise with the Saja Boys, casually encountering HUNTR/X in an alley. Shortly after, he is seen in a town square performing the song “Soda Pop” with the group. Later, the Saja Boys compete on a variety show in a hot sauce drinking contest. Mystery finishes in third place, where he is silenty brought to tears. After the show, he is seen with the rest of the Saja Boys at a male bathhouse, where they lure HUNTR/X into fighting water demons. After gaining popularity, the Saja Boys return to the Demon Realm to report their progress to Gwi-Ma. Mystery is shown making finger hearts as an idol before switching into his demon form, looking visibly exhausted. In his next appearances, Mystery is seen with the boys as they take home awards from music shows. Following this, he shown wearing a sleeping bag before the Saja Boys hijack the HUNTR/X fansign. When the two groups join tables, Zoey immediately flirts with Mystery when he sits next to her. Later, he barks at fans without warning and is reprimanded by Zoey for his behavior. Although he appears touched by the flower bouquets given by fans, he discards them in a trash bin immediately after leaving the venue. In his final appearances, Mystery is seen backstage with the Saja Boys at the International Idol Awards. He later performs with the group at their Namsan Tower concert, singing "Your Idol" in demon form. When Rumi interrupts the show to reunite with HUNTR/X, Mystery is shown flying toward the girls and engaging in a brief fight with Zoey. After Jinu's sacrifice, Mystery is the first to attack the other HUNTR/X members. However, he ultimately escapes by teleportion which is seen represented by pink smoke. She does manage to briefly see his uncovered face before delivering the final blow. Mystery's height and physique are nearly identical to the other Saja Boys members; he has a lean build with long legs and arms. His most distinctive feature is his long, lavender hair with bangs that completely cover his eyes. He appears to have a small nose and mouth, along with a sharp, V-shaped jawline. In demon form, he retains most of his human features, with the main differences being his tusks, purple skin, clawed hands, and patterns across his body. In his human form, Mystery is typically seen wearing sweater vests over mockneck shirts, earrings, arm warmers, plain colored pants, and sneakers. In contrast, his outfit in the Demon Realm consists of a traditional black jeogori jacket worn beneath a jeonbok overcoat, fastened with a sejodae sash, and completed with a gat. When performing "Your Idol", he wears a gat and sochangui overcoat with modern clothing