Aya, Yoji, Ken, and Omi are the four young men who comprise Weiss Kreuz (WK), an assassin group who eliminates certain menaces to society. Each member has his own trademark fighting style. Aya is adept at fighting with a katana, Yoji has mastered the art of strangulation via strings, Ken is lethal with his metal claws, while Omi specializes in a wide array of projectiles. Under the command of a mysterious man known only as "Persia", the WK is dispatched to take care of drug rings, terrorist factions, and the like. But as things go along, they discover that most, if not all, the bad elements they have dealt with are somehow connected to the Takatori family. The Takatori family is rich, powerful, and influential. It seems if the problem is to be nipped in the bud, the WK would have to put an end to one of Japan's most prominent clans. 
(Source: AniDB)

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__Non-anime roles:__  Takeshi Yamato in "IZUMO2 学園狂想曲" (Visual Novel)  

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Koua is an academy that brings Japanese's most talented people and students together. The mission is to train talented people to bear the world's future leadership. However, recently, the suicide rate in the academy is increasing and there is hardly any information being released to the public. 
Fujimiya Aya is sent to infiltrate this school carrying out criminal investigation and disguise himself as a teacher. In fact, actually, there is a connection between Koua academy and the global terrorist activities that are frequently occur. Therefore Persia assigns Hidaka Ken and Kudou Youji as well into this mission to solve the truth and connection of the incident behind Koua academy. 
(Source: AniDB)

Someone out there is determined to see the fall of Weiss Kreuz, and it looks like they have found the perfect plan for it. The four assassins find themselves trapped in a deadly game of deceit, where the question of honor versus duty comes to surface. Now they must fight for their own reasons to survive--and there can be only be one victor in this field of death. 
(Source: AniDB)

Weiss - an underground group of assassins composed of four men torn by their pasts; Omi Tsukiyono, Aya Fujimiya, Ken Hidaka and Youji Kudo. Each carrying the scars of their memories, they skillfully purge the city of criminals who have either evaded the law or are out of its reach. One by one, they begin to discover the relations of the crimes to something much more sinister, reaching out to their pasts. But each of them must decide if fighting for what they believe in and for the ones they love is worth it when it might come at the expense of their humanity...
 
(Source: Anime News Network)

The storyline continues from several months after Wei&szlig; Kreuz Gl&uuml;hen. It follows Aya and Ken as members of an assassination team named Krypton Brand Side B in Great Britain. Wei&szlig; is no more. Of the original Wei&szlig;, only Ken and Aya have found their way into Side B. 
From Mangaupdates