F/N L/N was found as a child by Ozpin, the young cat faunus clinging to a sheathed sword in an orphanage. Upon further inquiry, the staff said that he was found passed out on a shore near a lake with nothing but the weapon and the clothes on his back. Ozpin offered to take him in for reasons he did not specify.
Eventually it was discovered that the boy did not have an aura and semblance in the typical way. He had both, but his aura was less than a tenth of the strength of an average person. What he got in return for this weakness, however, was one of the most powerful semblances anyone had ever seen.
He could instantly move any way he wanted in two different ways. One was typical teleportation, and it only required that the location he was moving to was in his line of sight. The other was what really made him dangerous, allowing him to move either parts of his body, or all of it a short distance, but with the added side effect of pushing anything in its path out of the way. It seemed to simply always exert enough force to either break the obstruction, or send it flying, while putting no stress on whatever part of his body he was moving. This could be channeled into his weapon, allowing him to cut through almost anything that did not have an aura with the sword.
Upon finding all of this out, Ozpin formally adopted the boy, deciding that such abilities were worth getting on his side. As he grew up, he was trained to use his semblance to counter his weakness, either by dodging or deflecting the attack with his power. His weapon eventually received upgrades that included a dust tube being installed behind the blade, allowing him to channel whatever type of dust through it. This allowed him to send long-range waves of energy at his enemies, allowing him to forgo a gun. The dust type could be reloaded or switched by sheathing his sword and turning a dial.
Overtime, Y/N became one of the most dangerous fighters Ozpin had seen in years. His reflexes had greatly improved, and he was able to deal with large opponents that would have been trouble for others easily. He did, however, struggle to fit in with others his age, so he was not sent to a typical huntsmen school.