Personally, I agree with the Melancholy Bard, who seems to have been forgotten in transit, to a point. RPGs are defined (well, good RPGs) by the player's capacity to choose his actions and the subsequent results of those acts.
However this doesn't nullify the existing constraints; a character can certainly point at a random NPC and expect them to follow him into the jaws of certain doom, but the NPC won't have to. Actually, most would probably be calling the guard by that point...
Based on what's been said so far, Valen is a vampire who enjoys her power, and is more than willing to indulge in her darker desires, so any "redemption" quests would be difficult, if not impossible, except
for Bodhi's demise, but someone else has already covered that angle.
Then there's the defanging; also difficult, with Valen regularly reveling in her power, but not impossible. After all, she could be persuing some great power (maybe some artifact that renders her immune to sunlight?) and loose her undead status as a result. Yes, shades of Edwin. But Edwin delt with his change; who says Valen has to? Hm... feels like a mini-mod
though.
Romance has the dubious honor of being the most likely so far, as there's plenty of preciedent for it (Spike comes to mind...) Valen wouldn't have to outwardly like or even tolerate the PC, but unless the "Right Hand of Murder" plotline works out, she'd need some reason to hang around, and a crush (or confused emotions, or maybe all the free and exotic blood) is a
valid reason. Especially with Bodhi's death; purpose can easily be mistaken for love.
Now, on the flip side, Valen's effect on the PC should definatly weigh in. A "damnation" quest would be greatly appreciated, and would be quite easy to pair up with a redemption quest; Valen, trying to control her bloodlust seeks out some deus ex machina cure, only to get jumped by a pack of Paladins. The PC, who'd been helping her is forced to watch as she is cut down/staked/made to eat italian food/whatnot and goes Slayer. The Paladins die in a suitably bloodly mess, and the PC, bitter over the forces of light murdering a repentant soul (at least as far as the PC knows...) slips closer to CE. Particularly potent if the PC was a nieve Paladin... yes, yes, never happen; Paladins and Vampires don't get along.
Vamparisim would be an interesting addition as well, so long as Valen stays alive, and in control. Otherwise the PC gains ungodly powers in exchange for staying out of the sun. While Valen stays alive, the game might well become a power game between the two, with her giving orders while it's at the most inconvenient. Although, I'd imagine that would take
some considerable skill to implement and still make the game enjoyable.
And then there's the possiblity of the PC developing a crush on Valen, or perhaps even full blown love. That'd make a neat dream sequence now that I think about it... and it opens up the option of Valen toying
with the character before breaking PC's heart. Hm... depending on just how evil Valen is supposed to be, the PC could end up murdering everyone else in the group to keep her love, and *then* she decides to leave.
Basically, it seems that all lasting relations with Valen would be built around the question of control; Valen might very well want someone to command her right after Bodhi's demise, simply because she's used to it;
but Chaotic Evil is all about doing what YOU want, consiquences be damned, so she's lible to take power when she can.
Offhand, I'd like to see Anomen and Valen develop something, just because of the inherent passion such a conflict would bring, especially if both leave their discussions unsure... but once again, that won't happen.
Damn.