Krauxis, welcome!

Hello again Ark, first – sorry for the delay, I was abroad and didn't have much time to myself.

Second, congrats for the CS – it felt a bit off-story at first but I think I know where you're headed with it and I'll be here to help you adjust it to the story. :) Thus I'm not going to ask you to edit anything but I'll tell you what the best explanation is for your character to be like that and If you don't like it, just tell me. :)

So, the things that are a bit abnormal are his ability to manipulate essence (demons and humans are capable of it but then it's generally called magic with essence as the raw force needed to achieve a goal but only Hazumi (and the newly established musicians can manipulate essence itself), his ability to change essence (what you described he does through the runes) and him having an instrument before actually coming to the Academy.

Now, don't panic, I've already found a way to make that fit. What I suggest is following:

When my character (Hazumi, back then in the shape of a magical sword) was shattered (by Luciana), for her power/ fragments of herself to have scattered around like shrapnel. That allows people to have found them independently and have some skill even at the start of the RP. 

What Hazumi does from then on is find whoever has the fragments and invite them for formal training at the Academy – what could have happened with your character and how he joined in the first place. 

Third, Ark's essence manipulation shouldn't have been possible in previous seasons, however since Hell just broke loose and magic spilled over the human world freely, things are more chaotic as they were. That alone wouldn't be enough to allow humans to alter the very nature of things but it would make them more susceptible to (capable of) magic and the handling of such. This, combined with the fact he has an Instrument, justifies how he can make flames burn longer and such. 

Realistically you don't need to change anything, I'm just giving you some more info so you know exactly what you're playing and how unique your character would be compared to some just 20 years ago when almost none of this would have been possible. 

Also, for everyone, do keep in mind the world is in turmoil – the past 20 years have been heaven for a sheltered few (mostly Judas' followers or subjects, some of which are now hunted down mercilessly) and hell for many others with angels and demons rampaging on the Surface and a lot of genocide and war episodes.