A member of Shisio family shall be living with ayakashi power. Gen Shisio is the selected member for his generation. Yet, he didn't know what does the power had. All he knew, people called him as monster, so does himself. Even worse, after hearing his lovely sister agreed to put Gen, her only brother she just promise to protect and loving him, to special organization that handle person with monster power. Her words makes Gen gone wild uncontrollable, even hurting one of Night Troops of Shadow Organization who have been sent to pick Gen. Before the Chief handle with Gen, the brother had already hurt her own sister. Feeling regret, Gen Shisio became more wild. His sister can be cured and Gen can be taken thank to the Chief of Night Troop. The next morning he open his eyes, he found himself as Night Troop Headquarters member officially.
That's the past and background of Gen Shisio role on Kekkaishi story. Though he need no worry to being discriminated as Ayakashi-Majiri (half ayakashi-human), his unsociable attitude made no friends to sharing time together except Night Troop Headquarters trainer. No so different with his childhood. On Night Troop Headquarters Gen also called as monster as himself, because he still labile controlling his ayakashi-power, which mean Gen can be wild anytime. But, memory of hurting her sister can't just be forgotten. He train his controlling ability as complete any given mission, the only way make good use of his power. Even the missions take his body got badly injured, he will complete the mission. Until the Chief said that he's isn't type that can continue without mission.
Meeting with Karasumori's kekkaishi made him become more friendly (only to both kekkaishi). Some little mess happened, as someone mention about Gen past, especially about him almost killing his sister. But, friend came after some fight. The relationship on both kekkaishi and Gen getting better. Until new enemies trying to provoke Gen as monster as unwanted person. Struggling with enemies words and faith on his partner, Gen almost fall to enemy's trap. So he made up his mind to help and protect Karasumori's kekkaishi as friend and as partner. Gen now have mastered his controlling ability, transforms to ayakashi-form without losing his mind, stable. More strength to fight he gained by kekkaishi's word of faith.
Just the time he had full-control of ayakashi-power, the moment became his last performance. Too bad the enemy are stronger. Shisio Gen got killed tragically. The kekkaishi's trying to heal their partner. Gen said, he want to apologize to her sister and the Chief for being disable to continue the mission to protect Karasumori land with both kekkashi's. One of kekkaishi is crying, Gen ask why. Then he saw the picture of his memories, he's ready to died. Without regret, Gen Shisio died in peace with smiling face.
Gen's death affect much on some characters. The Kekkaishi boy. He got blamed because he didn't know that by Gen transformed, it does risk his life, moreover he wanted to revenge Gen's death on enemy whose killed Gen. Gen's sister. She always waiting for his brother's back, yet she got him in death body. Then blame the Chief for his lack causes Gen died. The trainer. She is the only person on Night Troop Headquarters who Gen can speaks and show emotion. Same with kekkaishi boy, she has burning spirit to finish the enemies off.