Astro Boy was created by Doctor Tenma, and was meant to be a replacement for his deceased son named Tobio (Toby in various English translations). However, because Astro was not able to grow and age like a normal human child, Doctor Tenma abandoned him to a Robot Circus led by the cruel Egg Ham.
At the circus, Astro Boy was found by Professor Ochanomizu and adopted. Professor Ochanomizu gave Astro a family with a robotic mother, father, sister (Uran) and brother (Cobalt). As head of the Ministry of Science, Professor Ochanmizu often calls on Astro to help resolve situations involving humans, robots and (on occasion) extraterrestrials. The team called Astro "Atom" because it can mean "Atomic" robot Despite his abilities and work for good, Astro is often ostracized by the human race for being a robot. Since he is not officially considered a citizen, he often has to deal with the prejudice of humans against robots. Ultimately, Astro has to find a way to bring peace between humans and robots.
(From the Astro Boy wiki)
Megaman Zero was leading a squad of soildiers to attack Copy X. "Are we even close?"one of the soildiers complained. "Soon we will be." Zero responded. As soon as he saw X's fortress, he ordered his soildiers to attack. They opened fire once they were in. Copy X , unknown to Zero, teleported right behind him an teleported to the Hamegg Arena. He found himself behind what looked liked a gate. He looked outside the gate and was horrified with what he saw. He saw robots fighting to the deah and being destroyed without any sense of honor. Then he saw Astro Boy. He immediatly thought he was a reploid.After seeing him fight some robots, his thoughts changed. He then heard Hamegg announcing "Somene named X has sent robot called Zero to compete in these games. He heard the crow chearing. The gate opened as he stepped out. "And luckily, we had another robot that would be competing called Astro Boy!" Hamegg roared. 'I have to fight him?' Zero thought. Zero and Astro Boy prepared to fight. Zero took out his Z-saber and charged at Astro Boy. Astro flew into the air and started shooting Zero with his cannon arms. Zero retaliated by dodging and quickly took out his buster shot and charged it to it's maximam. He shot and it hit Astro's foot and he fell to the ground. Astro then used his finger lasers at Zero and Zero took out his shield boomerand and blocked the finger lasers. He then threw the Shield Boomerang and sliced of the finger laser guns. Zero then activated his Zero Knuckle and used beam shot him, which hit Astro in the leg, causing him to loose balance. Zero then jumped and used Widebuster and sent a barrage of missles to hit Astro. Astro just notices them and he barely dodges out of the way. He is on the ground and Zero walks towards him cautionly. Then the Butt Machine gus come out and Zero is hit by the blasts and is hit by the blasts. He then brings out the flame sabre and hits the ground, sending a barrage on flames at Astro boy andit hits Astro,causing damage. Zero then comes at him with his Z-saber and Asto gets ready to fight. Zero slashes at Astro and hits him in the arm. Astro tries to punch Zero, but he activated his Death Pierce and stabbed Astro with it. Zero then sensed his blue core, and realized he can only kill him by taking away the blue core. But before he can do anyhing, Astro punches him down and prepares to kill him with his arm cannons. But the, Zero activates his Gyro Boomerang and knocks him away. As Astro struggles to get steady, Zero activates Bee rod and it goes through Astro's chest and Blue Core is on the end of the rod. Astro quickly falls down dead. Zero looks down at his dead opponet and wondered if their was an alternative.
This was a close match, both warriors strong in their own way. But Megaman Zero's superior agility, more expiriance and better training gave Zero the win. Not to mention Astro Boy's weaponry is more for taking on groups of and bigger opponets, where as Zero'a weaponry as for 1 on 1 battling.