First and foremost, as in all these cases, Tess is hot, Isabel's hot, and they look damn good together. There are of course many other reasons why the pairing is attractive. One that stands out is the great chemistry between them, especially in the early episodes. From the moment they met they clicked, before Isabel had any idea of Tess' origins – almost as if they were destined to be together. Isabel defended Tess when all the others were suspicious of her. "Four Square" was by all accounts, and judging from scripts and screencaps, bursting with subtext, which continued on and off. The revelation that Tess was one of them was especially important, and in the dreams, they stood opposite each other; I have this weird thing about symmetry, and how that should mean something. But leaving that aside, Tess and Isabel are the only two female aliens in the world (leaving aside the Skins, the jellyfish crystal queen, any other aliens-of-the-week they decide to insert, and Lonnie and Ava, who are the same people anyway). Which naturally gives them a lot in common. They share the same race and the same difference and the same secret... which they have to keep together. Hey, it's just like being gay – have you noticed how all these teen-angst sci-fi shows are one big gay metaphor? I digress. Back to the point: Tess is also the only one of the aliens that Isabel hasn't regarded pretty much as family her whole life, making a relationship with her less incestuous-feeling than one with Max, or to a lesser extent, Michael. (No offence to Outsiders or Cliffhangers, I support them too.)

Isabel certainly cares a lot about Tess, as shown most evidently in "Surprise". She's frantic with worry and thinks nothing of driving across country and ruining her very nice dress to save her. She saves her life, then, and protects her, holding her in her arms in a very emotional rescue scene. Not only that, but she kills Whitaker for her, which is a pretty intense reaction. It also seems significant that Isabel is the one Tess is calling for. Never mind Max, who's her husband, she calls for Isabel and Isabel hears her cries, which is a pretty strong suggestion that she's the most important person in her world.

Of course, in "Departure" it's all over. Tess' true identity is revealed as an evil midwarping hell beast – excellent move by the writers there. Not. Sadly it's left to the fans to fill in the gaping holes in canon, and there has to be a reason for Tess' turn to the dark side. A "pact that Nasedo made 40 years ago" is an immediate explanation, but there has to be more to it: there's no reason why she'd go through with it. Perhaps it's the lack of love and the rejection that must have followed her throughout her life, from Nasedo's cold upbringing to the Roswellian's unwelcome attitude. Maybe Isabel not loving her was the final straw. OK, that may be over-interpretation, but it's interestting that she chose Alex to do her dirty work, he being Isabel's sometime-boyfriend and therefore her rival. She also kissed Max at the prom, which happened just after Alex and Isabel had got back together.

But even the events of "Departure", regardless of their reasons, also provide further reasons for Mindwarping being a valid ship. Isabel was Vilandra, who betrayed her family and planet; this time round Tess is the traitor. So they can identify, empathise, and have been shown to be yet more alike. The Tess-betrayal storyline could have been done so much better; as an example, see Faith in Buffy, whose descent into evil (and eventual rehabilitation) was made fascinating by great writing and acting. The two situations are actually fairly similar, especially if you believe in Buffy/Faith, as I do. But no, Tess went evil in about two episodes with zero explanation and zero exporation of the consequences. Well, that's Roswell for you.

So there we have it – two beautiful (once) blonde teenage aliens, willing to kill for each other, opposites in the Royal Four, both betrayers, kept apart by cruel fate. Which all adds up to a perfect match.