Gen Shishio the Heartless Transfer Student

Gen Shishio the Heartless Transfer Student (非情な転校生 志々尾限, Hijō na Tenkōsei, Shishio Gen) is the 21st episode of the Kekkaishi anime adapted by Sunrise.

Summary
Two mysterious strangers visit Karasumori Academy at night. The taller one warns the younger one (who bears the Shadow Organization star on his uniform) that he must control himself. The next day at school, Yoshimori and Tokine visit Tatsumi Mino in his office to warn him that Ayakashi disguised as humans may sneak into the school, and that he should report anything suspicious to them. Mino is largely unconcerned and takes the chance to reintroduce them to his snakes, which have learned to talk due to Karasumori's influence. The same young stranger from the previous night (now wearing a school uniform) passes by the office, giving it a suspicious glance.

Yoshimori and Tokine continue to discuss the new Ayakashi outside Mino's office, and Yoshimori insists that Tokine call him for help (as opposed to Mino) if she finds one. Tokine realizes that he is maturing and no longer a crybaby. Later, Yurina notices some of her classmates messing with Yoshimori's hair while he's asleep, but she is distracted by Kyoko and Ayano, who are excited about the mysterious new transfer student. Yurina goes with them to check out the new guy, but passes a boy in the hallway (the very same young stranger) and notices that he smells like blood.

Back in class, Yurina decides to tell Yoshimori about the new student. She drags Yoshimori up to the roof once class ends, but Yoshimori repeatedly asks her to tell him about it later in a dull, robotic tone and walks away. Before he can get back to class, the new student drags him back outside. Yoshimori speaks to him the same way, so the new student loses his temper and punches Yoshimori's head into a wall, revealing him to be a Shikigami (though a thoroughly destroyed one, now).

On the roof, Yoshimori is startled from a nap as he senses his Shikigami's destruction. The new student transforms his hand into a large claw, releasing an evil aura in the process. Sensing the aura, Yoshimori and Tokine rush to the spot, but only find a huge set of claw marks in the wall. They discuss the situation while they begin to restore the damaged wall. Yoshimori is concerned because the Shikigami was supposedly a major improvement over his usual ones, and confused because he felt the evil aura after the Shikigami was destroyed, as if the Ayakashi wanted to be found. Tokine suggests that they are being purposely provoked. Angered, Yoshimori rushes off, sending out several Shikigami birds to track down what he believes to be an Ayakashi in human skin.

Yoshimori comes across a group of high schoolers that have been beaten up, and learns that the new student is the culprit. One of the Shikigami spots the new kid harassing Mino, and Yoshimori rushes to help. Mino is alone when Yoshimori reaches him, but confirms that the student had amazing power and knew that Mino had a power as well. Yoshimori senses another Shikigami being destroyed and runs off. Two more Shikigami are snatched from the air before Yoshimori spots the new student on a nearby rooftop. After destroying the Shikigami, the student runs, and Yoshimori chases him, with Tokine joining the chase soon after. Suddenly, the student turns and attacks. Yoshimori barely dodges the first few blows, but is quickly overwhelmed. He manages to trap the student in a Kekkai, but the student's hand transforms into a claw. Tokine arrives just in time to see the student tear through the Kekkai with no effort. He introduces himself as Gen Shishio, a Night Troop member sent to assist them. However, he is greatly disappointed in their lack of skill.