Hakata Tonkotsu Ramens

At first glance, the Fukuoka city seems like a peaceful one, but under the surface, crime is running rampant. The city's Hakata ward is home to the professionals of the underworld: professional killers, detectives, informants, professional revenge seekers, those skilled in torture, and more. And according to urban legend, even a "killer of professional killers" exists in the city. When stories of the men in the underworld are told, a "professional killer murder" appears.

(Source: Anime News Network)

From Endless Journey:  
An assassin from Kitakyushu appears. Hakata: a town rumored to count 3% of the population as killers. Saruwatari, a dangerous man in search of thrills, hears the rumors of a "killer of professional killers"  and comes to this battleground for professional killers. Add in Nitta, a hitman consultant, and trouble may arise in Hakata once again!

A collection of short stories across the whole time line. Each chapter is an individual story and can be read separately.

Ikebukuro, Tokyo. The strongest killers from Hakata arrive in the city of Ikebukuro where unique individuals gather!

Zenji Banba and his partner Xianming Lin come to visit Tokyo City to see a baseball game when an unfortunate predicament leaves them with none of their possessions and money. Needing to make quick money, they take the request to hunt down a bounty hunter. Along their adventure in the ward of Ikebukuro they encounter the local residents, from the normal average high schooler or rowdy bounty hunters to the city legends that are the mischievous informant from Shinjuku and a super-human man in a bartender uniform.