Oh Todd. I do feel a little bad for him in many of the books, because despite the fact that he and Elizabeth are always being described as the most perfect couple ever, they're pretty far from it.
I can think of seven guys she's cheated on him with off of the top of my head, and I know there's gotta be a few more. Elizabeth seems to think it's important to "explore her feelings" for other guys every now and then, even though she's sooo in luv with Todd.
Case in point: Deceptions. Elizabeth goes on a date with rich, hot Nicholas Morrow, because she feels like it's only fair to make sure there's no spark between them, and give Nic a chance. So basically, she's testing potential boyfriends when she's supposed to be committed to Todd. Nice. Todd forgives her when he learns that she doesn't really like Nicholas and takes her back. No big deal!
This pretty much is repeated again and again, and a good summary of Liz and Todd's relationship. They have a fight in nearly every book (often about what a doormat Liz is when it comes to her twin. Usually, he's right.), and a sappy ,make-up kissing marathon in the end.
Todd moves away from Sweet Valley in Say Goodbye. He comes back in Brokenhearted, richer then before. He's in private school until the next book, In Love Again, when he returns to SVH. Naturally, he and Liz get back together. Barf.
Towards the end of the SVH series, it appears that they're done for realz this time, but I haven't read all of the Senior Year series yet, so who knows if they get back together again. The suspense kills me!
Todd is way more interesting in the Sweet Valley University series. He deals with an alcohol problem and some criminal charges! Oooh.