Kyodai Ken

"Gotten a second wind, have you? Good! This will make your defeat all the more satisfying."

- Kyodai Ken aka The Ninja.

Kyodai Ken (also called himself "The Ninja") was a skilled ninja and an enemy of Bruce Wayne.

History
Kyodai Ken was once the star pupil of Yoru, a top martial artist in Japan, but his code and ethics were dishonorable and he often used these skills to steal. Kyodai found a less-than-worthy rival in the form of a foreigner named Bruce Wayne. One night, Kyodai attempted to steal Yoru's prized sword but was confronted by Wayne. Yoru investigated the commotion and expelled Kyodai from his 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 Wayne. He began committing several stings of corporate sabotage on seven Wayne Cosmetics branches, but Batman eventually 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 Wayne, and indirectly, Summer Gleeson after a Gotham Charities Reception.

Wayne couldn't fight Kyodai or he would compromise himself in front of Gleeson, who was investigating Batman's secret identity. Fortunately, Robin followed Kyodai and restricted Gleeson's view, allowing Wayne to fight and overpower Kyodai. Kyodai, instead of surrendering, dove into a nearby river and escape.

Months 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 (Oonemuri / great sleep) Touch, which was capable of killing a man in a single blow. Kyodai kidnapped Yoru's student Kairi and blackmailed him 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.

He kidnapped Alfred Pennyworth and lured Wayne to Mount Kijiki for a final showdown, when Wayne discovered his hideout. Kyodai used the touch, but it failed, because Wayne wore a protective pad to defend the death point. When the volcano exploded, Kyodai was stranded in the melting rocks. Wayne attempted to save him, but Kyodai didn't oblige. Respecting his opponent, he bowed to Wayne before another explosion of lava came between them and Kyodai had vanished.

Kyodai's final fate is unrevealed, however, as he was not seen or heard from again, it is more than likely that he perished in the explosion.

Background information
Bruce Timm has since admitted that they had no plans of bringing back Ken 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 Kyodai was killed in the blast.