Kyodai Ken

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

- Kypodai Ken aka The Ninja.

Kyodai Ken (also know as The Ninja") was a skilled ninja and an enemy of Bruce Wayne from Batman: The Animated Series.

History
Kyodai was a good student of one of the best martial artists in Japan Yoru but he was dishonorable. He encountered and rivaled Bruce in Japan during his training as Batman. He consistently beated Bruce at martial arts. One night, he tried to steal a prized sword of Yoru but was faced by Bruce before they could fight, Yoru saw what was happening and expelled Kyodai from the temple. Kyodai's pain was was so great that he resettled in the city, somewhere in the Uramachi District. Kyodai named 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.

Trivia

 * 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.