For why I roleplay, I suppose I got into that through wanting to play tabletop RP, but I never managed to get a game together and IRL I'm shy enough that I never could get into character the way I wanted to. Then I got into Star Wars Galaxies and EvE, and realized that no one was ever going to see my face and that I could do what I wanted, which was really freeing for me, and really fun. Which probably says something about my mental state, but whatever.
As for how much of me is in my characters, it varries. Saikoyu is probably the most like me IRL, shy, bad at fights (verbal or physical), etc, but she is a lot more passive than I usually am. All of my alts that I RP with are usually based around Sai, so there is less of me in them, and usually I just started of awith a key concept and then based the rest around either actors in shows or people I know. A'kua is based a lot on several Belly Dancers I know, and Chantel is basically a female version of Captain Jack from Dr. Who/Torchwood, but without the angest, yet anyway.