Character Analysis
Probably the first question you asked after picking up a copy of the book is, what's the deal with all the Katherines? Shmoop is with you. The fact that every girl Colin's ever dated is named Katherine is more than a strange coincidence, so let's think about what it means. Here's what we're told about the Katherines:
When it comes to girls (and in Colin's case, it so often did), everyone has a type; instead of physical preferences, though, Colin Singleton's type was linguistic: he liked Katherines. And not Katies or Kats or Kitties or Cathys or Rynns or Trinas or Kays or Kates or, God forbid, Catherines… just K-A-T-H-E-R-I-N-E. When we meet Colin, he has dated nineteen girls, and every single one of them has been named Katherine… and all of them—every single solitary one—had dumped him. (3.26)
Are you sensing a pattern here? It goes beyond the shared name, though. Colin is so concerned with himself that in every relationship the only thing he seems to really remember is the fact that he was dumped. His relationships with Katherines are all just clones of one another, and the only relationship he seems to remember specifics about is with K-19.
The Katherines are more a symbol of Colin's view of women and relationships than they are about any single woman, which is why they're all described at once near the end of the book. It's also why Colin can't remember if he was the dumper or the dumpee, and it's definitely why he refers to them by number (K-1, K-5, and so on… super unromantic, if you ask Shmoop).
In short, all the women in his life are just copies of one another, and Colin's playing out the same relationship again and again, until finally he breaks the cycle with Lindsey.