Saiyuki is a prime example, as is Araiso Private High School Student Council Executive Committee, which is pungently funny. The two main characters from that anime, Makoto Kubota and Minoru Tokito, appear as the central characters of the manga series Wild Adapter. The two have been used to seed a new series, and a fairly dark one, and their relationship has become even more ambiguous. Araiso is quirky and mordant; Wild Adapter is dark and bitter.
Kubota is recruited by the Izumo gang to run its "youth division." He's cold-blooded and seemingly unconcerned with most thing that concern most people. He's a born gambler, and he always wins. And he's only in high school. Tokito is running from someone, but who that might be is a mystery to us. This volume is mostly about Kubota, so we only catch glimpses of Tokito's story, but we do see that he wears a glove on his right hand, which he loses fairly quickly. That hand is furred and clawed.
This volume is set-up for another series. The central motivator is a series of strange deaths, caused, apparently, by a drug overdose. The victims have all been transformed into beasts of a sort, and no one knows where this stuff is coming from. It's called "Wild Adapter."
The series is pretty episodic, so you can, if you wish, jump in any place, but this first volume gives some very helpful basis. Minekura has built a fair amount of complexity into Kubota's character here, not to mention some vivid portraits of the supporting cast, although Tokito is pretty much a mystery still.
She also maintains her in-your-face attitude, and she's playing with a boys' love element. Kubota's boss in the gang wants something more than a lieutenant, although Kubota, typically, is not interested. That element will resonate strongly in the relationship between Kubota and Tokito as it develops later in the series.
Minekura tends to occupy a territory somewhere between shoujo and shounen manga: narrative is fairly straightforward, although she doesn't tie herself to regularly shaped frames. The drawing style can only be described as hers: quirky, based on a bishounen aesthetic, sometimes startlingly expressive, and ultimately very appealing.
It's Minekura diving headlong into the dark side of human nature. I've finally picked up volume 5, and volume 6 is due out soon. There are episodes that aren't so appealing, but in general it's worth the time.
It's from Tokyopop.