According to Kuroh Sakamoto's account of events Ken was created by GUiDE after the events of the 2011 Outbreak of Alpha Worms. Discovering a man by the name of Kanbe Ogawa was naturally resistant to the alpha worms his DNA was taken and this immunity isolated and amplified resulting in a cloned "son" that would become known as Ken Ogawa.
Unfortunately the experiment was a failure in that in addition to being toxic to the Alpha Worms Ken's blood was also toxic to regular humans as well. Making the most of the situation Ken was trained in the way of the Shinobi by his "father" and Methods of accelerated aging were employed to grow Ken to adult hood in under four years.
As a adult Ken is unaware of his unusual heritage and upbringing beyond the fact that he was born into the Ogawa clan and has easily befriended the other members of his unit including Andy Walker the team's pilot and Kuroh Sakamoto his rival.
Ken's unit is dispatched to Tokyo while in the midst of the largest Alpha Worm outbreak on record. While jumping out of the helicopter Kanbe gifts Ken with the clan's 'Ninja Blade.' Ken alongside Kanbe are dispatched to deal with a infestation in a burning building where they defeat a Level 4 Carriers Pox Giant using the Cyclone Shuriken before they are forced to fight a Arachne. After being given a crash refresher course on his weapons Ken kills the Arachne with a wrecking ball.
The victory is short lived as Kanbe and Kuroh slaughter the entire unit, including Andy, revealing that they're both infected by King Worms. Ken attempts to kill the two traitorous members of the unit with the Ninja blade but Kanbe takes it, unlocks it, and runs Ken through with it, missing his heart by millimeters. After less than a single day of rest Ken is dispatched back to the city alone to deal with the infection.
Ken is informed that with the severity of the infection GUIDE's top brass are seriously considering "glassing" the city via orbital laser cannon, which will kill every thing in the city including the uninfected majority of the population. Ken starts his task with investigating reports of what could be a level 5 carrier. With out so much as his feet on the ground Ken's chopper is attacked by a Scolex Worm.
Commandeering the Chopper's gun Ken fends the worm off before crashing. After fighting through a hoard of enemies include a couple of Impaler Leech's and a Slime Chopper Ken eventually finds the source of the local infection: a Plague Snail mysteriously empowered by one of his clan's Sacred Gem's. Killing the Snail Ken appropriates the gem and uses it to add the fire element to his shuriken.
After dealing with the Snail Ken is informed of two King Worm hosts nearby. After tracking them down Ken discovers that they're a father daughter pair of Yakuza bosses Ryoko and Oyabun Tojiro Kurokawa. The pair attempt to break Ken off of their trail by feeding him to a Carrion Claw, a massive electrical crab like creature that through the use of electrical Gem is shooting lightning.
Once the Carrion Claw is killed Ken appropriates the gem for his Shuriken. After another brutal chase Ken confronts and kills the two bosses, crushing Oyabun under a highway and Ryoko under a steam roller, Ryoko regaining some of her sanity begs to be killed to end her pitiful existence. The encounter gives Ken some hope that he can save his father from his infection.
Ken is informed that a group of flying Carriers are attacking a Jumbo jet full of uninfected evacuees, apparently trying to spread the infection overseas, using level 2 explosive fly carriers to bust a hole into it. While fighting the Level 5 carrier leading the attack, a Fire Hydra, Ken is met with his father again who questions if he's ready for the threats he faces.
Apparently deciding that they can't hijack the plane any longer they decide to crash it in the middle of Tokyo, having already infected everyone on-board. After killing the Hydra Ken lands the jet on a highway in the middle of Tokyo, nearly killing a group of survivors in the process. Agent Michael Wilson calls and informs Ken that they've located Kuroh who has attacked the local shopping mall.
Ken goes to the shopping mall where Kuroh sics a Blast Mite on Ken. Ken defeats the Blast Mite and true to it's name it explodes before he can kill it sending him plummeting into the tunnels below. Kuroh informs Ken that he has placed three more Blast Mites in strategic locations throughout Tokyo's underground each one ready to level a City block. After eliminating two of the Blast Mites Ken fights the final and largest of the three: a Stalker Larva.
Recognizing that killing the creature underground will set it off and take a large portion of Tokyo with it Ken forces it into a service catapult and forcibly launches the explosive carrier into the air above Tokyo where he safely kills it without it doing any harm to the city. Unfortunately Ken soon learns that the bombing was meant to be a distraction while he leads a strike team of Carrier military personnel into GUIDE HQ.
As a crowd gathers around the Tokyo branch of GUIDE where as one Reporter notes that although the building has been taken over with hostages no demands have been made and the authorities have yet to respond. On que Ken crashes a APC through the front door of GUIDE HQ and immediately begins fighting RPG launcher totting Carriers.
Learning that Kuroh has taken hostages on the top floor, including the director Michael and various other GUIDE personnel. Once at the top Ken faces off against the reanimated corpses turned Carriers of his former team-mates attacks him, most notably Andy all of whom Ken puts out of their misery. With Michael's help Ken is able to save all the other hostages and gets them to safety.
Once on the roof Ken and Kuroh fight once more, as he dies Kuroh mockingly reveals that Ken was not born but made, in a lab with the singular purpose of wiping out the Alpha Worms. Further more Kuroh also reveals that both he and Konbe infected themselves with the King worms on purpose, desiring more power that could not be achieved with a body that is "just human."
Deciding to kill Ken and take himself with him Kuroh makes use of the explosive charge inside his body in a suicide attack on Ken. Konbe saves Ken from the explosion and partially confirms Kuroh's story: yes he did infect himself with King Worms, so that he could infiltrate and locate the weak spot of the hive, which he has. He two also warns that the Alpha worms have buried the bulk of their hive deep underground to avoid the glassing.
Following Kanbe's information Ken goes underground to see the hive for himself. Once his APC is destroyed Ken falls into a mystical trap that extracts his Mystical shinobi gems that fly off and augment three high tier carriers: a Carrion Claw has his electrical gem once more, a Pox Giant has his flame gem and a pair of clones of Ryoko and Tojiro has the cyclone gem.
Reclaiming his equipment Ken soon makes his way to the sewage treatment plant where he encounters another Arachne. After dropping the Arachne into a processing fan Ken discovers that not only is the Hive where Kanbe said it was but according to Kanbe it's also huge the hive extending beneath the streets of Tokyo. Ken attempts to use the Ninja Blade to cut down the hive only for it to electrocute him, as he is not ready yet.