My nickname started back when I was about 10 or 11. My friend's one day started calling me "Hikari" because my real name (Amber) means light.
That stuck and I tried to use it on the internet in later years. It was a difficult task, however, as Hikari was a popular name used and I had to add things to it to make names like Hikari-chan or Hikacchan and so forth in hopes of lucking out. I also went by Hikari Hatake sometimes. I didn't like using these honorifics with my name seeing as it felt strange to add a title others should use, not you.
Eventually, I started creating my artist name and so forth and decide to change it around a little bit. And so, "Hikari" became "Hikarin" (光鈴) [light+bell] because it was a nice bit of imagery for me and was still Hikari, just with an n on the end of it. Also, -rin is also another honorific and could be taken that was as well without it seeming strange.
It's a little complicated and round-about, but I have really grown attached to it now. ^///^
I also use "Senevir" (pronounced sen-eh-veer) now on non-asian-related sited. It's a name my boyfriend and I came up with for use in roleplay.
Before I started using Hikari on the net, I also used Lenaay, Kiki-ra and KyotoGirlsomethingorother too! xD