[@ZxyxZemon] The Zoldyck have a rather extreme way of teaching Killua has a lot of training that my character doesn't. Lyle's god-father taught him to transmute his ability by trying way of meditation, and by teaching him how friction works by having him learning how to start fires with various substances. He basically learned it by figuring out how to get his nen to mimic the way fire works. It isn't so much that Lyle is immune or incapable of being hurt by normal fires so much that his own fires don't hurt him. When his Hatsu is active fire does hurt him less but he's not all together immune to it. Its not a great explanation but it is the best I have for the moment. If this is not sufficient an explanation I can try and swap fire with ice or rock instead being exposed to the elements and all could have had him learn to work with those elements. 

This all works in tandem since his god-father who I intend on having his nickname be 'The Great Bear' uses his own Nen to physically enhance his body being a enhancer type. Lyle basically taking his core concepts and applying it to his own while using a natural affinity of his own to make it his own. As for growth Lyle has room to grow in manipulating, and controlling his own aura that isn't immediately outside of his own body. A way to make reliable ranged attacks, to turn his fire into things like say a fire made Javelin and just hurl it someone. Or to turn it into something more utility based like super heating a door, or melting something instead of just having it coat his body like protective armor since as I stated above its basically his nen/hatsu taking the quality of fire and applying it itself.