"Marcastr0", sometimes stylized as "MarcAstr0", is my username pretty much everywhere, so I thought I might as well share its origin.
Where I went to university, like in most higher education institutions, students and faculty were given an email address and a username to log in to the school's computers. To come up with the usernames, the system administrators at my school created an 8-character long string made up of pieces of the student or faculty member's first and last names. With my name being Mario Castro, the result was "marcastr" (apparently other options like "macastro" were already taken).
I became so used to being referred by my username (not just me but a lot of my classmates as well) that I decided to use it elsewhere, but to make it more pronounceable, I added an "o" at the end, so it became "marcastro", or "Marc Astro," which I thought sounded cool and I ended up using it as my "stage name" in my musical endeavors. To make it look more "hacker" or "1337", y changed the last "o" for a "0" (zero), thus, resulting in "marcastr0."