Hajime Shishizaki

Hajime Shishizaki is a character mentioned in Durarara!!x9 who has, so far, never made a physical appearance in the series.

Characteristics
Almost nothing is known about Shishizaki in terms of his personality or his overall appearance. In terms of physical abilities, Shishizaki is said to rival both Izaya Orihara and Shizuo Heiwajima in agility and strength, respectively. He is also apparently well-trained, as there are rumors saying that a US soldier who was vacationing in Japan was beaten by Shishizaki in a fist-fight.

Background
What little is known about his history before the series is that he is the son of a rich family and that he attended Raijin Academy with Shizuo, Izaya, Shinra Kishitani, and Kyouhei Kadota. He was popular at school and was a relatively good student. It is said that he would frequently get caught in the battles between Shizuo and Izaya, although it is unclear if his involvement was intentional or accidental. He is said to have been fast enough to catch Izaya in parkour (which Shizuo was unable to do in their teenage years) and strong enough to stop Shizuo during his rampages. Once, Celty saw people being punched into the sky from a distance and thought she was seeing Shizuo, but when she walked closer, she realized it was not Shizuo but someone else (Shishizaki).

In the Shizuo-cenric gaiden released with DVD vol 2, during high school, Izaya joking tells Kyouhei, "Haha, if Shizu-chan is the Shadow Boss, you definitely are like the Light Boss," to which Kyouhei replies, "Not interested. It's true we don't have any senior who could take that title. Still, if anyone should be the Light Boss here it would be Shishizaki-senpai. Either way it has nothing to do with me."

After he graduated Raijin Academy, he apparently worked at Russia Sushi for a short while before either quitting or getting fired. Afterward, Shishizaki eventually left Japan to study abroad in America.

Dragon Zombie Arc
Shishizaki is mentioned in passing by Shizuo. He remarks that he was one of the few people who could keep up with Izaya's raw agility, saying "The only one who could catch him was Shishizaki back in high school...."

Durarara Side Stories
Bits and pieces of Shishizaki's life are revealed in most of the Durarara!! side stories that were published in various magazines.

Shizuo Heiwajima
Shizuo refers to Shishizaki as his senpai and seems to hold some degree of respect for him.

Izaya Orihara
Izaya appears to have a negative impression of Shishizaki, as he gets visibly annoyed when he is mentioned. When rumors start to circulate about Shihizaki's return from overseas, Izaya appears to be visibly disturbed.

Trivia

 * Since the Durarara!! side stories are the only consistent source of info on Shishizaki, fans had for a time referred to them as the "Shishizaki monthly."
 * Light novel volume 9 is the only reference in the main series to Shishizaki's existence.