I made my first screen name when I sf2 was popular
and my fav was Ryu
Well then I played a game where you had to pick a last name
so i picked masters cause that was Ken's name and it was the best I could come up with (there is actaully a long story involving me running up my mom's phone bill calling capcom headquarters in whatever asian country it was, but we'll leave that alone)
so I had the name Ryumasters forever because Ryu was almost always taken anyways, but got tired of typing the whole dame thing out so shortened it to Ryum and have had it ever since..
It is never taken on mmo's, or pretty much anywhere. It is short so people can type it with ease, not like people with 25character names or like lillliliIllIIllll and the minimun on more games these days is 4 letters so its all around useable