Hajime Shishizaki

Hajime Shishizaki is a character mentioned in Durarara!! light novel volume 9 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 was 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 Shizuo and Izaya had although it is unclear if his involvement was intentional or accidental. He is said to be fast enough to catch Izaya in parkour (which Shizuo was never able to do in their teenager years) and strong enough to stop Shizuo during his rampage.

In volume 9, Shizuo is quoted as saying: "Only Shishizaki-senpai from our high school could catch up with him and grab his collar..." in reference to Izaya.

From a distance when Celty saw people being punched into the sky, Celty thought she was seeing Shizuo but once she walked up closer, she realized it was not Shizuo but someone else [Shishizaki].

In DVD vol 2 Shizuo Gaiden, during their high school, Izaya joking tells Kyouhei, "Haha, if Shizu-chan is the Shadow Boss, you definitely are like the Light Boss." to which Kadota 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, 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.

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. Other than that, little is known about the two of them.

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 have started referring to them as the "Shishizaki monthly."
 * Light novel volume 9 is the only reference in the main series to Shishizaki's existence.