However, as the Dark Spore developed and continued to influence him, Ken became heartless towards society, viewing everyone around him as pathetic insects beneath his notice. He even became the star player of the Tamachi Soccer team, where he broke the record for scoring most goals in a league season with 45. From there, Ken adopted several aspects of Sam and very rapidly developed into a child prodigy, excelling perfectly at virtually everything that he chooses to pursue, whether it's an intellectual activity or an athletic one, such as chess or judo. Doing so caused the Dark Spore to activate, and Ken began believing that the Digital World is a place for him to shape and command as his own. There, Ken dipped his Digivice in the ocean, turning it into a dark D-3. Ken did this and was transported to the Dark Ocean. Some time later, an e-mail from a mysterious figure (revealed later to be Yukio Oikawa) told him to use the Digivice to escape the guilt and sadness over Sam's death. Ken was devastated following the accident, suffering from strong feelings of guilt because he felt that he'd somehow caused Sam's death by his 'wish'. Tragically, Sam died after being hit by a car shortly after that day. As a result, Ken again found himself wishing that Sam 'would just disappear'. When Sam came back into his room and found Ken holding the Digivice, he scathingly berated Ken for breaking his trust and proceeded to kick Ken out of the room. Wormmon and Ryo tended to Ken for three weeks until he was well enough to return to the Real World, where he found that it's the same day as he had left this is likely due to Millenniummon's ability to warp time and space. Ken pushed Ryo out of the way of the incoming Spores, and one of them embedded itself in the back of his neck. However, when Millenniummon was defeated, he forced out pieces of his data in the form of Dark Spores. There, he met his Digimon partner, Wormmon, and helped Ryo Akiyama in fighting Millenniummon. When Sam left the room, Ken, in childish curiosity, touched the Digivice and was pulled through Sam's computer into the Digital World. It was meant for Ken, but neither of them knew this, so Sam picked it up and put it in his drawer, telling Ken not to touch the drawer until he could figure out what it is. One day while with Sam in his room, a Digivice came out of Sam's computer. However, because of everyone's attention being focused on Sam, he also finds himself jealous of his older brother, sometimes wishing that he was an only child. At times, Ken got along very well his brother and even showed admiration for him, especially when they went out to the balcony to blow bubbles together.

As a young girl, Ken felt ignored by his parents in favor of his older sister Sam Ichijouji, who was a child prodigy.

Ken dark spore