Kishimoto Seishi manga
In a post apolyptic world devoid of seasons, where man's "golden age of technology" ruined earth and caused science to run amok, there live four individuals looking for a door to the hell far below the surface. Through a series of coincidences, the ex-hitman-now-bartender Mihai, the deranged and psychopathic Haine, the vengeful yet kind Naoto, and the freelancer Badou meet one another.
Sometime in the future, somewhere in Europe a city has many levels of underground, even secret dangerous levels that have answers to the past. Mihai is a retired assassin who has returned home to settle a vendetta and start a new life as a restaurant owner. Haine was genetically experimented on as a child, which led to him possesing dog like attributes. Badou is his clumsy chain smoking partner who becomes deranged when denied cigarettes. Naoto was orphaned as a child and has amnesia she was trained to wield the sword. Their pasts are somehow connected to the secret underground and the organization that controls it.