often seen in the street without a hat

Not exactly, Francine Pascal is a real person (and I think it's her real name, though I may be wrong), and she has written several books, including some of the early SVH ones. However, the various Sweet Valley books are written more along the lines of the Stratemeyer syndicate (the people behind Carolyn Keene/Nancy Drew books and others), in that Pascal writes a vague outline, and then ghostwriters write it out along the familiar lines. Kate Williams is the person who fleshed out the outline for Jessica Quits the Squad!. I do believe, though, that Kate Williams, and Laurie Johns (who wrote the early SVUs) and the others are group pennames, and the real writers behind those are sometimes the people who the books are dedicated to. However, I don't know why I think that, and I am prepared to be told that I have made it up entirely.

I like knowing pseudonyms too! I think it's because I was shocked when I was little and found out that Martin Waddell was also Catherine Sefton. And I liked knowing about the Bells/Brontes.