Yu-yu Kondo begins his story as a shy, third-year middle school student in Kanazawa city. He possesses short dark blue hair, fair skin, and purple eyes, typically dressing in a green and white jacket, black jeans, and brown laced boots. Pressure from his older sisters frequently forces him into cross-dressing. These outfits include a honey-blond wig with a black headband, a white and green vintage dress with a black ribbon, or the Raika Academy female uniform: a dark pink blazer, white blouse, grey vest, pink bow, knee-length dark grey skirt, and low-heeled shoes. Later, his signature jacket becomes a cropped, unzipped red and black design with a similar pattern, worn with a white shirt, black jeans, and red zip-up boots.

Initially timid and prone to stammering, Yu-yu struggles to assert himself, especially around his sisters, making him easily manipulated into situations like public cross-dressing. He possesses the unique ability to "read the hearts" of others, sensing their emotions and underlying feelings. However, this initially reinforces his reluctance to voice his own desires or refuse requests. His journey into cardfighting sparks significant personal growth, gradually enabling him to express his opinions, stand up for himself, and protect others. He fights desperately to safeguard Team Blackout's banner during Danji's absence and takes initiative to help Tohya regain the team's trust.

Megumi Okura invites Yu-yu to Dreamland Wonder Hill, an abandoned amusement park serving as Team Blackout's base, where he first encounters Vanguard and joins the team. Team Blackout functions as a found family united by their love for Vanguard, welcoming anyone with good intentions. Following Danji Momoyama's departure to reconnect with Mirei and Tohya Ebata's resignation to travel, Yu-yu assumes leadership of Team Blackout during high school. He attends the same school as Raika Koshiba and later passes leadership to Megumi Okura. After reaching an understanding with Gui, Yu-yu departs Team Blackout to travel and explore unseen parts of the world.

As a cardfighter, Yu-yu uses Dragon Empire decks centered on evolving mechanics. His signature unit is Trickstar, a Grade 0 unit representing his potential and desire for growth. His initial deck utilizes the "overDress" ability, evolving Trickstar into various Vairina forms (Valiente, Arcs, Exspecta, Esperaridea) for versatile effects like power increases, retires, draws, or re-standing. His primary vanguards include Chakrabarthi Divine Dragon, Nirvana, and later Chakrabarthi True Dragon, Mahar Nirvana, which recur Trickstar from the drop zone and apply pressure through damage-dealing abilities. He later adopts the "X-overDress" mechanic, combining Trickstar with Prayer Dragon units into forms like Vairina Arcs and Stravairina, supported by vanguards like Chakrabarthi Phoenix Dragon, Nirvana Jheva. He strategically integrates both overDress and X-overDress units, enhancing synergy with order cards like Clad in Prayer and incorporating new units like Mirror Reflection Equip, Mirrors Vairina to counter ideological opponents. His deck continuously evolves, adapting with new support units and triggers.

Yu-yu's cardfighting record reflects his development. Early losses occur against Danji and Tohya, but he eventually achieves victories against Megumi Okura, Katsuki Murayama, Shinobu Kajita, and Mirei Minae. Key wins include his initial encounter against Tohya and a victory over Mirei at Nagoya Castle, where he uses his emotional insight to break through her defenses. In tournaments, he loses his first match against Raika Koshiba but wins their rematch. His final duel against Gui employs a deck focused entirely on X-overDress, culminating in his departure after victory.