Would it be legal to use someone's online username in a book?

Names can't be copyrighted, although they can be trademarked, which would allow the owner to stop anyone from using it without permission. It's highly unlikely that anyone would bother trademarking a username, unless they were already using it in other contexts or it was worth a lot of money to them (say, they got a lot of advertising revenue from a blog associated with the username).

The only way you're likely to run into problems is if you state or imply that your character is the real-life person behind the username, as they might decide to sue you for libel and/or invasion of privacy. If you just like the name and are using it for your own character, you'll be fine.

EDIT: I sometimes use the username/nickname "pembers" on forums and websites, and it annoys me when I find someone else has got it before me on any particular site. But since it's obviously derived from my surname, I can't really claim any kind of exclusive ownership over it.

No you should be okay. The most they might try is to sue for intellectual property. Which are things like books, art, speeches, even hairstylist for a movie or something

Basically works that are hard to physically show for but exist in context.

You could always contact a law firm that offers a free hotline where you can ask a lawyer directly.

I've been using Roadwalker for over 10 years in different boards and have an established blog called Roadwalker. If someone took my username I would try to stop them.

I don't know how other users might feel, but maybe the same.

You can use any username you want, even if someone else came up with it. Usernames cannot be copyrighted.

Steven J Pemberton I really need your help with my question.