These are really silly theories I had in my mind so they might be grasping at straws or whatever and you may have permission to laugh at me
Axl was created by Gate.
- Axl praises X and Zero very highly. Gate has always wanted to get a hold of their DNA, he may have created Axl to do so, due to his Copy Ability. However, this could just be Axl looking up to them because of them being "legendary"
- Axl is "younger" than X and Zero are, he may have been built around the time Gate invited Zero and X to his Laboratory.
- During the Sabotage and Maverick labeling, Axl was scrapped by Gate, then later found by Red.
Lumine took a liking to Axl, and White Armor theory
- No, I dont mean in a romantic kind of way. Lumine sees potential in Axl, so he planted the "crystal" in Axl's forehead to enhance his abilities, which is why he is able to copy Mavericks and things larger than himself.
- White Armor also allows Axl to control Copied states longer than usual.
feel free to rip me to shreds :p