Naritaverse

Naritaverse is the unofficial fan name given to the shared universe created by Durarara!! 's author, Ryohgo Narita. Naritaverse encapsulates ''Durarara!! along with three other novel series by Narita: Baccano!, Vamp!, and Etsusa Bridge''.

Baccano!
The earliest entry in Naritaverse, Baccano! tells the stories of various immortal beings and the chaos they bring to themselves and those around them. The story arcs take place in a variety of different time periods, from the early 18th century to the Great Depression/Prohibition era of the United States to the turn of the millennium. It is the longest running and most expansive series of Naritaverse.

Vamp!
Taking place in modern-day Germany on the fictional island of Growerth, Vamp! tells the story of vampire twins with unique properties, their bizarre adoptive father, the island's dhampyr mayor and citizens, and many other vampires, humans, and variations of the two that get dragged into the story.

Etsusa Bridge
Chronologically last in Naritaverse, Etsusa Bridge is set in the late 2010s and early 2020s. It takes place in a city built on a man-made island located in the center of a massive bridge between Sadogashima Island and Niigata. The bridge's construction was abandoned last minute, becoming a haven for outlaws and outcasts.

Anime

 * Isaac Dian and Miria Harvent, a pair of eccentric thieves from Baccano!, are present at the first Dollars meeting in Ikebukuro. (Episode 11)
 * Isaac and Miria are also seen discussing "Japan's ghostly fireballs" (Hitodama in the original sub), who are actually people covered in glow paint thrown across the sky by Shizuo. (Episode 12.5)

Light Novel

 * In volume 4 of Durarara!!, the serial killer known as "Hollywood", who wears various monster costumes while committing murders, is compared to an American couple known for committing robberies in costumes. The couple mentioned are Isaac and Miria of Baccano!


 * Nile, one of the original immortals from the Advenna Avis, somehow got into a fight with Shizuo while on vacation in Ikebukuro. The fight ends in Shizuo's favor.
 * The unnamed photographer who narrated volume 01 of the Baccano! novels apparently lives in the apartment next to Mikado.
 * Ronny Schiatto, a high ranking member of the Martillo Camorra as well as the 'demon' who bestowed the crew of the Advena Avis with immortality, is apparently an acquaintance of Celty's. However, the details on how they met aren't known.
 * During the modern-day story arc, Bride mentions "Orihara of Shinjuku" as one of his primary information sources. This is an obvious reference to Izaya.
 * Izaya also appears during the aftermath of the hijacking of the cruise ship the Entrance as it docks in Japan. He assists Bride in disposing of the bodies of some of the hijackers.
 * In volume 8 of Durarara!!, Saki makes a comment on how the child stunt actor, Charon Walken, bares a striking resemblance to Kasuka. It's worth noting that Charon and his sister Claudia  are the great-grandchildren of Felix Walken(formerly known as Claire Stanfield) and Chane Laforet.
 * A popular director in Baccano!, John Drox, frequently hires Kasuka for his movies.

Anime

 * Shingen explains his work with Nebula to Walker, mentioning an "elderly video game-obsessed loli" and a "beautiful hungry-type werewolf girl." These are references to Romy Mars ("The Silver-Wheel Stage") and Watson respectively. (Episode 4x01)

Light Novel

 * Vamp! is one of the only series to have a full on crossover with Durarara!! Appropriately titled "Duramp!," the short story follows a pair of succubi (demons who feed off the lustful desires of men) as they journey to Ikebukuro for their next meal.
 * The first succubus visits Walker in his sleep and shape-shifts into one of his greatest fantasies, a green haired maid. The next day, Walker attempts to tell his friends about how said maid had visited him the night before but everyone else just dismisses it as his usual ramblings.
 * The second succubus chooses Shizuo as her next target and decides to take the form of the most prominent person in his mind. Unfortunately for her, at the time, Shizuo had just gotten out of another fight with Izaya and the encounter was still fresh in his mind. Mistaking Shizuo's blind hate for blind passion, the succubus takes Izaya's form and attempts to flirt with him. Thinking Izaya had returned for round two, Shizuo savagely beat the succubus into the ground and went home. The succubus, meanwhile, returned to her previous city horrifically traumatized.
 * A famous martial artist in Vamp!, Traugott Geissendörfer, owns a series of martial arts schools across the world. Mairu Orihara learned most of her martial arts skills from the Japanese branch of one of his schools before eventually moving to Rakuei Gym. On a side note, Shizuo is also a big fan of Geissendörfer, who was invited to speak at Raijin High School's closing ceremony during Shizuo's graduation year.
 * A famous artist from Vamp!, Carnald Strassburg, is also briefly mentioned by Shingen while solving a crossword puzzle that required the artist's name for one of its answers.
 * Also, a fake lithograph of one of Strassburg's paintings was attempted to be sold to Chikage at an art gallery in Ikebukuro.
 * In a discussion about various vampire clans, the Hijiribe clan is mentioned as a small family network that has assimilated into human society. This ties in with Ruri Hijiribe's and Kasane Kujiragi's partial vampirism.
 * Shinichi Tsukumoya, a character who appears in a handful of Durarara!! novels, is one of Izaya's primary information suppliers and serves as a narrator. It is implied that Shinichi does not exist in the real world but only on the internet as a sort of AI. A character in Vamp! called QAWSED ("Hackey Mouse") also claims to only exist on the web as part of a race of beings who live in digital form. It is likely, though not outright stated, that Shinichi is a member of this same race.
 * According to Narita, Manami Mamiya and Masashi "Mage" Mamiya of Vamp! are somehow related, though the nature of the connection is a secret.

Light Novel

 * It is said that Celty and Shinra have something to do with the political scandal caused by the construction of the bridge. This is also mentioned in the x2 Shou OVA.
 * Aoba is seen conversing with an unknown Blue Squares member at various points whose last name is Yatsufusa. This is the surname of the original producer of Buru Buru Airwaves. After his death, one of the Etsusa Bridge main characters, Kelly Yatsufusa, took up his position as DJ and adopted his last name as her own.
 * Souji Kuzuhara, head of the West District volunteer police, is part of the Kuzuhara family of police officers including Kinnosuke Kuzuhara and Shinju Kuzuhara.
 * The Ei family of the Dragon Zombie bosozoku gang is the same Ei Family as the Western District Chinese mafia.

Yozakura Quartet
While not part of Naritaverse, Yozakura Quartet and Durarara!! have connections due to their shared illustrator, Yasuda Suzuhito.
 * A 100-page crossover manga titled "Yozakura Quartet x Durarara" was released as a bonus for the home video release of "Yozakura Quartet: Hana no uta." The exact plot details are unknown at this time so it is uncertain if it is considered canon.
 * In volume 03 of the light novels, Izaya instructs Namie to send several documents to a Sakurashin trading partner in his computer's address log.
 * Masaomi Kida is seen singing Sakurashin Town's anthem in one of the chatroom conversations in volume 04 of the light novels and directly mentions Sakurashin Town as the origin.
 * Assuming this is the same Sakurashin Town from Yozakura Quartet, and due to the large demon populace of said town, this reference implies that the existence of demons and other supernatural creatures is common knowledge in the Naritaverse.
 * If this assessment is true, a minor plot hole exists during the first Dollars meeting when Celty reveals her true form. Many people were surprised or even horrified at Celty's appearance. Assuming demons are common knowledge, a headless horseman riding around a city shouldn't come as this big of a shock.
 * It is possible that Sakurashin Town isn't well known to the world at large and as such, while people may know the town exists, its large demon population could be kept secret from the general public. This would make Ikebukuro's response to Celty's nature a bit more believable.
 * Alternatively, it is possible that, while demons are confirmed to exist, they are extremely rare outside of Sakurashin Town and as such it would mean most humans have very little experience with the super natural which, again, makes the public's reaction to Celty more plausible. This also makes Celty's relatively quick acceptance into normal society more believable as, after her big reveal, all that occurs are a few news reports and an increase in police patrols. Were Celty the first supernatural creature to go public, it would have attracted much more attention than a mild increase in traffic patrols.