Hideo Harada is the head of the Shiranami Karuta Society and a central mentor figure in the competitive karuta world. He is a middle-aged man who works as a medical doctor, balancing his professional life with his deep passion for the game. He is commonly seen in the brown robes of his karuta society or in casual brown jackets, with black hair and glasses that frame his bespectacled brown eyes.

Harada is portrayed as a man with a kind heart but a fierce and overwhelmingly competitive spirit, especially when it comes to karuta. His personality is marked by deep, insightful knowledge of the game, but also a tendency toward dramatic over-exaggeration, as seen in his effusive and boisterous welcome of new players into his society. He is tenacious and believes in an aggressive, offensive style of karuta, a philosophy he instills in his students. As a mentor, he is their staunchest supporter, yet he is not afraid to deliver harsh truths to push them forward, such as when he told Chihaya she needed to abandon her over-reliance on speed to become a more complete player.

As the leader of the Shiranami Society, Harada plays a pivotal role as a teacher. He first encountered the series' protagonists, Chihaya, Taichi, and Arata, when they visited the society as children. He quickly recognized Chihaya's natural talent and has continued to mentor her and Taichi throughout their middle and high school years. He affectionately nicknamed Taichi Eyelashes-kun due to his long eyelashes. Beyond teaching, Harada is an active competitor still striving for the highest honors in the sport, serving as a living example of lifelong dedication to karuta. His own dream of becoming the Meijin is a significant subplot, showcasing that passion is not limited to the young.

Harada's relationships extend beyond his students. He has longstanding rivalries with other figures in the karuta world, including Kitano, the head of the Suihoku Karuta Society, with whom he has been on bad terms since their youth. Conversely, he maintains a friendly rivalry with Kuriyama, the director of the Nagumo Karuta Society.

Throughout the story, Harada experiences significant development as he continues his own pursuit of the Meijin title. He has been forced to adapt his playing style, most notably creating a technique called Sudden Stop Acceleration specifically to counter the tactics of the reigning champion, Suou. This technique modifies his usual aggressive, full-swing motion into a controlled stop to avoid committing faults. This drive has been a long time coming; his backstory reveals that he once had to put his dreams of glory on hold to focus on his medical career and family, as karuta could not provide a stable living. His return to serious competition marks a personal victory and an attempt to finally challenge for the title that eluded him in his youth.

With regards to abilities, Harada is considered a master-level player. His style is defined by relentless aggression and intimidation. He possesses several unique talents. One is known as Joker Card Detection, a sort of intuition or superstition that allows him to sense which card on the board will never be read during the match. He combines this with a specialization in endgame scenarios, becoming exceptionally dangerous when there is only one card left on the opponent's side. It is said that when he was younger, he also possessed superior hearing, a talent that mirrors Chihaya's own exceptional auditory abilities.