Liz
Some of the more observant fans of Roswell will know that there is a slight possibility that Liz could be the fourth alien. It is true that she could, but she was naturally human. Max healed her and accidentally converted her to the alien side. It becomes obvious that she and Max are actually destined to be together. This becomes easier to understand, for in the last episode, Graduation, he proposes to her and she accepts.
Read more about this topic: Fourth Alien