I have decided to confirm that the flashback section of Chapter 13 is officially the prologue of the K: BTS series, thus making it Chapter 0 in a sense and before anyone asks why that Chapter wasn't the first one made, the answer is that I originally did not have a background story for the story or the characters when I began writing the series with a group of friends back in 2008.
When I was posting the comic here I DA, the thought hit me that people would like to know how the characters became the people who they are now and I want to point out if a character mentions something that has happened to them in the past or a flashback is shown, this is obvious meant to be canon to the comic. Also, I want to show many other things in the series as mentioned many times that it takes place in it's own universe.
So, now on to what inspired Chapter 13... I had a close friend of mine, Aaron, who I consider my brother staying over at my house since he had no where to go due to a previous relationship of his didn't work out. Anyway, on his birthday, he got a Xbox and Naruto Shippuden Ultimate Ninja Storm 3 and he said I could play it while he was at work. I did and this song that played in this boss battle with Kurama, aka the Nine Tailed Fox, caught my fancy. The battle inspired me to write Chapter 13 and I listened to this song many times while drawing the Chapter. So, yes, Naruto also has a bit of inspiration for K: BTS as well.
This is the song right here