why did you leave ashLet's see, I guess I were about four and I was deeply in love with the "Misty of Chincoteague" novel which my mother regularly read to me. My papa (father) took to pet-naming me Misty among other nicknames and over time that one became the most common.
Around 11 when I got on the interwebs, I used other avatar names including Misty, but around 15 when I started playing the League of Legends beta, I was lucky enough to get Misty as my name. From there, I got involved in a clan that played the game pretty regularly and it just stuck as my online name. My papa still calls me Misty pretty regularly too, so I do associate it pretty closely with myself.
That said, I'm pretty used to people assuming it comes from Pokemon. It slightly annoys me, but I see why they do it.