Dr Mikoto Nakadai/ Abarekiller : he was a famous surgeon, who was very competent in his job, but felt completely bored with his life. However, the day the Bakuryuu came in Tokyo and began rampaging, he felt for the first time an incredible feeling of excitement when he did witness the destruction they've caused. That same day, he's had the opportunity to operate a young man who had been seriously injured because he tried to save a dog (namely, Ryouga Hakua/Abared). Even if he managed to save his life, he was so disgusted by that behaviour that he didn't finish the surgery, leaving the rest to his assistants. Soon after, he witnessed the Bakuryuu being calmed down by several people (including his last patient), and soon his feeling of excitement faded. But just as he was giving up, an object fell from the sky, and he saw a big egg, and a wrist brace. He soon realized that a creature inside the egg was communicating with him, and he took the brace and tried to use it. At first, all he was able to feel was big pain, but, after a while and training, he's been at last able to change into a white suited warrior, to his delight. After witnessing the Abaranger for a while, he decided to confront them at last. After defeating (without killing him) a Trinoid (Yatsudenwani), he introduced himself to the 4 heroes. At first, the Abarangers hoped that he would join them in their fight for justice, but Nakadai, calling himself Abarekiller, told them that he wasn't interested and would rather use his powers to have fun and play "games". That "fun" included beating the Abaranger with quick and vicious attacks, and collecting their Dino Guts. He uses the Wing Pentacle as his weapon, and can create arrows or other stuff to attack the heroes by drawing them with it. He needed Dino Guts to revive his partner, a very dangerous Bakuryuu called Topgaler. At the same time, Asuka realized that Mikoto was wearing a prototype Dino Brace that was still flawed, and could explode any time, and causing intense destruction. However, Mikoto didn't care, and even felt it made the game more interesting. Soon, despite the best efforts of the heroes, Nakadai has been able to revive Topgaler, and they soon began their destructive partnership. Soon, Stegoslaidon couldn't bear ir and tried to attack the devilish duo, but he was soon under the control of Topgaler, and fused with him to form a big robot, Killeroh, who soon attacked Abarenoh, in order to take control of the heroes Bakuryu. Nakadai almost succeeded, if not for Stego regaining control of himself and separating from Topgaler. However Stego has been tempted by the incredible power that he could display with Topgaler as Killeroh, and soon willingly joined them.
Of course, the Evolian have taken notice of Abarekiller, and realized that he got the prototype they thought they have lost when the first Anomalocaris was destroyed. Of course, they wished to get back those powers, but in vain, since Abarekiller and Killeroh were powerful opponents.
At the same time, Ryouga found out that Nakadai was none other than the doctor who saved him when he was injured. He wanted to thank him, but only found disdain and contempt from Nakadai, who instead started to fight him, mocking him at the same time, and taking delight at seeing Ryouga's suffer. Indeed, Ryouga, who was so optimistic, believing so much in the good in humans, was confronted with a true psychopath, who didn't care at all for his fellow humans, was taking fun in hurting people and doing evil, and was completely in contradiction with the way he felt people should be. The fact that that man was the surgeon who saved his life made it even worse.
Soon, Abarekiller often confronted the Abaranger, notably to take control of Bakuryuu to gain more power. He managed to capture Bakikeronagurus and Dimenocodon, and trapped them in barrel with a bomb, and began a cruel game, when he challenged the Abaranger in finding a Trinoid (that he's been able to obtain after a phone call with the Evolian) and destroy it, in oreder to be able to save their friends. The cruelty of the game was revealed to be even higher, when Ryouga realized that Nakadai had corrupted a idol singer trio and their sponsors into getting success thanks to the power of a trinoid, even if it means inducing violent behaviour in children. It completely shattered Ryouga's mind, who was helplessly witnessing the worst tendencies of human nature. That psychological blow was even bigger after they failed to free the two Bakuryuu from Abarekiller. Even if, instead of killing them, he just merely decided to control them, it was a devastating defeat for Ryouga and his friends.
However, Mikoto didn't join the Evolian; he was merely using them, and he didn't mind fighting them, as well as he fought the heroes. At times, he even helped the Abarangers fighting the Evolian, when he was annoyed with the evil clan. Besides, having to control more Bakuryuu had painful consequences for him, and he often had to rest and try to overcome the pain.
One day, he was visited by a little girl, who was none other than Rije form the Evolian, who took an interest in him. At first, he didn't care for her, but when he realized that she had the power to link the two Earths, and as such, allow him to go to Dino Earth, he accepted to spend some time with her, allowing her to have some fun time with him, even if he himself didn't especially enjoy it; at that time, he has been able to use his Abaremode.
Soon, his patience has been rewarded and Lije took him to Dino Earth. Once there, he decided to become the leader of the Evolian, and fought three Trinoids (the so-called "Evoranger") in order to earn the position. Once the Evoranger defeated, he became the leader of the Evolian, to Lije's delight and Mikela, Voffa and Jeanne's dismay. Now being able to use the Evolian's resources, he created his own kind of evil monster, Killer Ghost, and, using his two captive Bakuryuu, lured Ryouga, Yukito, Ranru and their Bakuryuu partners in a island where Killeroh quickly defeated them and trapped Tyranno, Tricera and Ptera in his body.
However, even if he took great pleasure in tormenting the heroes, winning the game, especially so easily, didn't bring him pleasure, but boredom instead. When the heroes came back to fight, especially with Ryouga becoming Abaremax, and managed to defeat Killer Ghost and get back their friends, instead of being annoyed at seeing the heroes getting stronger, he felt that the stronger his enemies were, the more fun the game would be, especially when he could use all the Evolian's power to create even more crazy and twisted games to torment the heroes.
One day, he even had Voffa create a Jupiter Giganoid, who was able to attract Jupiter towards Earth, and, as a result, destroy Earth completely, including himself. When Ryouga witnessed Nakadai being so twisted, taking delight at destroying a whole planet by himself with total indifference for human life, he, who had never hated anyone, began to be consumed with fury and hate and fought Abarekiller as Abaremax, not wanting just to defeat him, but even to kill him, and he came very close to kill Nakadai, who nevertheless was rescued by Topgaler.