Obviously, Cyborg 004 is one of the main characters. There might also be the scientists or engineers who created him. They could play important roles in the story, perhaps as mentors or antagonists depending on the plot. Another possible main character could be a human who befriends Cyborg 004 and helps him understand the world better.
The information provided by the gunslinger TV series did not mention that the series had 40 episodes, nor did it mention that it could be watched for free. Therefore, I don't know if the gunslinger TV series has 40 episodes and if it's free to watch.