Kyodai Ken, also known as the Ninja, was a skilled ninja and enemy of Bruce Wayne.
Kyodai Ken was once the star pupil of Yoru, a top martial artist in Japan, but his morality left a lot to be desired. Kyodai found a less-than-worthy rival in the form of a foreigner named Bruce Wayne. A sparring session between Bruce and Kyodai ended with Kyodai the winner, who scoffed at Bruce's background. His boastfulness was put in check by Yoru, who proceeded to floor Kyodai with advanced moves, warning that there is always someone better and that a good martial artist is ethical and respectful to others. One night, Kyodai attempted to steal Yoru's prized sword, but was confronted by Bruce. Yoru halted the forthcoming fight, reclaimed the sword and expelled Kyodai from the dojo. Kyodai's pain was so great that he resettled in the city, somewhere in the Uramachi District. Kyodai contracted himself as the assassin for hire, the Ninja.
Years later, Kyodai traveled to Gotham City as the Ninja in order to exact revenge on Bruce. He began by committing several stings of corporate sabotage on seven Wayne Industries branches. While trying to break into Wayne Cosmetics, Batman intervened and when he displayed his stance, Kyodai realized Batman was Bruce Wayne. Batman also realized his identity as well, after a batarang from Robin cut the cloth on his back, revealing a menacing tattoo that Kyodai had during his training.
He kidnapped Bruce and indirectly, Summer Gleeson after a Gotham Charities Reception. There, he revealed his plot: while he hated Bruce for getting him kicked out of the dojo, he also hated that he was born with a silver spoon in his mouth. The purpose for his robberies of Wayne Industries was to hack into the finances of the companies then transfer them to his bank account. Summer said the thefts made him a petty crook, but Kyodai mildly scoffed at that, saying by all the subterfuge he had committed it made him a clever thief.
Bruce was able to escape after Robin had distracted Kyodai, but was unable to fight him as he would expose his alter ego to Gleeson. Fortunately, Robin restricted Gleeson's view, allowing Bruce to fight and overpower Kyodai. Instead of surrendering, Kyodai dove into a nearby river and escaped.
Later, Kyodai returned to Japan and became intent on learning Yoru's ultimate technique, the 牙の方向 (Kiba no Houkou / Way of the Fang)'s 大眠り (o-nemuri/big sleep) touch, which was capable of killing a man in a single blow. He kidnapped Yoru's student Kairi and blackmailed her into giving him the location of the hidden scrolls containing the technique instructions. However, time had made the scrolls fragile and they fell apart upon contact, but Kyodai still managed to learn the touch from a recovered fragment.
He kidnapped Alfred Pennyworth and lured Bruce to Mount Kajiiki for a final showdown, when Bruce discovered his hideout. Kyodai used the touch, but it failed, because Bruce wore a protective pad to defend the death point. When the volcano erupted, Kyodai was stranded on a melting rock. Bruce attempted to save him, but Kyodai, silently acknowledging him as a worthy opponent, bowed to him before another explosion of lava came between them and Kyodai had vanished.
Kyodai's fate is unrevealed, however, as he was not seen or heard from again, it is more than likely that he perished in the explosion.
Bruce Timm has since admitted that they had no plans of bringing back Kyodai in later episodes, and wanted to make his death more explicit, which was shot down by censors who pressured him to make it more vague. Therefore, it is probably safe to say that Kyodai was killed in the blast.