I cried like a baby when I saw the episode of Alex' death.
He is one of my most favourite characters of the whole show.
He has always been such a good and loyal friend, first just to Liz and Maria, and when he finally knew the truth about the aliens, also to them.
At the beginning, he was sort of an outsider, because he didn't know of the big secret. His friendship to Liz and Maria almost breaks until Liz tells him everything in "Heat Wave".
He has a crush on Isabel, but for her he's only a good friend for a long time.
After he comes back from "Sweden" he has changed a lot. But truth is that Tess mindwarped him and he was in Las Cruses to translate the book of the Royal Four. After his return, Isabel would have liked to be with him, but before a real relationship between them is formed, he's killed by Tess.