Alec essentially has two types. The first and longest-standing is dark-haired, elegant freaks with something Wrong with them mentally - Viola, Lynn, and Phoebe are all examples, but this goes for men too, and as I expand Alec's social life it's a pattern you'll see recurring again and again.

The second is people who remind him of Isabelle. Since Isabelle was a young child, Alec's been pursuing people who remind him of her so that whatever urges he can't take out on her he can manage through them. This ranges from simply sex (when she was very young and he wasn't raping her yet) to murder, and you'll get to see a fic about that sometime. Blonde hair isn't a must, but it helps a good deal, as do large eyes and a sense of whimsy.