DRRR!! character name trivia.
Ryuugamine Mikado （竜ヶ峰帝人）& Kida Masaomi（紀田正臣）
"Dragon Peak Emperor" and his "righteous vassal". This is intentional, at least according to Narita.
Orihara Izaya（折原臨也）, Orihara Mairu（折原舞流）& Orihara Kururi（折原九琉璃）
"Orihara" is not too uncommon a surname - Narita probably wished to stress his "ordinary origins" here. The kanji for Izaya's given name, a very rare combination if ever used at all, would usually be pronounced "Rinya" and definitely not "Izaya". And it seems that 臨, when used singly in a male name (a rare instance as well), is usually pronounced Nozomu (correct me if I'm wrong here!)
TV News: ...The injured, Mr. Orihara Izaya, is...
Namie (squinting at the screen): (It's definitely him. Though they didn't put a photo of him on TV, it's not like there would be a second person in Japan who would have that kanji combination as a given name and pronounce it as "Izaya".)
- Vol. 7 (Not a translation - I just kind of tried to sum it up.)
The only reason his name is pronounced "Izaya" was that his parents wanted it to sound the same as the biblical prophet Isaiah (phonetically "Izaya" in Japanese). When Narita first introduced Izaya he mentioned another origin of his name - 臨 is also a reference to the phrase "臨む者", which could mean "the one who rules" but also multiple other things depending on the context, e.g. in the following biblical quote:
地の上に空な事が行われている。すなわち、義人であって、悪人に臨むべき事が、その身に臨む者がある。また、悪人であって、義人に臨むべき事が、その身に臨む者がある。わたしは言った、これもまた空であると。 -伝道の書 8:14
There is a vanity which is done on the earth, that there are righteous men to whom it happens according to the work of the wicked. Again, there are wicked men to whom it happens according to the work of the righteous. I said that this also is vanity. - Ecclesiastes 8:14
Izaya himself commented in Vol.4 that his parents' naming sense was unusual to say the very least. "Mairu" and "Kururi" aren't ordinary names either. "Mairu" is used very rarely and almost exclusively as a pen name or handle. "Kururi" is a twist of the more mundane "Ruri", which could mean either Lapis lazuli or a type of Chinese glasswork.
Heiwajima Shizuo（平和島静雄）& Heiwajima Kasuka（平和島幽）
"Peaceful Island Serene Man". But you know that part already. What you may not know is that Kasuka's given name is just as unindicative as his brother's: it means "obscure" while everyone knows him as Hanejima Yuuhei the top idol.
Like "Senjougahara", "Heiwajima" is a place name (in this case an artificial island in Oota, Tokyo) instead of a real surname (again, native speakers - I got it from 2ch discussions so correct me if I'm wrong.) "Shizuo", on the other hand, is a fairly common male name and has a couple of kanji variations.
In Vol.3 Shinra explained to Celty the link between Kasuka's true name and showbiz name: the "Hane" in "Hanejima Yuuhei" can alternatively be pronounced as "Wa" and "Yuu" as "Kasuka"; that gives you "Wa-jima-kasuka-hei" - a rearranged version of "Heiwajima Kasuka".
Kishitani Shinra（岸谷新羅）& Kishitani Shingen（岸谷森厳）
Like Izaya, the Kishitanis have a not-uncommon surname but unusual given names.
"Shinra", alternatively "Shiragi", is the Japanese/Chinese name for Silla, a long-lived kingdom in ancient Korea.
Ironically, Shingen's given name means "solemn".
Celty Sturluson
We can guess that "Celty" is almost certainly intended to refer to her Irish/Celtic origins. "Sturluson", however, is a Nordic patronym (i.e. not an actual surname) whose most famous user was the poet and politician Snorri Sturluson, born to the Sturlungar family in Iceland.
Kadota Kyohei（門田京平）
His name is a reference to the renowned light novelist Kadono Kouhei, who published his most celebrated series - Boogiepop - under Dengeki Bunko.
Yumasaki Walker（遊馬崎ウォーカー）
It is implied various times in the series that he might be half-white - hence the "Walker". His name is a reference to another Dengeki Bunko light novelist, Okayu Masaki (author of the Bokusatsu Tenshi Dokuro-chan series).
Karisawa Erika（狩沢絵理華）
Her name is a reference to the Dengeki Bunko light novelists Arisawa Mamizu (Infinity Zero series) and Nakamura Erika (Double Brid series).
Togusa Saburo （渡草三郎）
Dengeki Bunko novelist Watase Souichiro (渡瀬草一郎) (Sora no Kane no Hibiku Wakusei series).