I know what you're thinking: an ensouled vampire?  Aren't they supposed to be incredibly rare?  Like, only two documented cases in all of history?  Well, yeah.  But like it or not, as a staple of the Buffy genre, having an ensouled vampire in the group immediately became a staple of Buffy gaming.  Anyway, it gets worse: he's also dating a Slayer.  Sort of.  You see...    

Darien didn't start the game with a soul.  Long before the Slayer population explosion of 2003 gave Alejandra Martinez super-powers to fight the forces of darkness, Darien was her long-time boyfriend. They were both each other's first... er... adult relationship, and it's possible they were really and truly in love.  He was the first person she shared her new destiny with, and he joined her faithful corps of Scoobies because he couldn't sit idly by.  Tragedy struck the day that he was taken captive by Alejandra's arch-nemesis, a vampire drug lord, and sired by him in a cruel twist of psychological warfare against the Slayer. 

Darien took to being a vampire with gusto.  Even as vampires go, he was a sick and demented freak.  Vampires always seem to be at their most dangerous when they have something to obsess over, and they have a pronounced tendency to be obsessed with the people they cared most for in life.  He swore to Al when he was alive that he would always love her, and he wasn't lying; he continued to love her in the morbid and creepifying way that only a vampire could.  He was also incredibly ambitious, and he had a plan.  And not a bad one, either. 

He meant to keep the Slayer alive by any means possible.  He looked out for her, he sent her warnings by way of a reliable snitch, and made sure he gave her information she could trust without ever finding out who it came from.  He even staked a couple of his master's other minions when they got too close to doing her real harm... but not so anyone found out.  He played the obedient lackey while biding his time.  Don't mistake this for altruism, which being a vampire he wasn't strictly capable of, because he would have beaten her half to death and then drank her blood given half a chance, but only so he could make her a vampire. 

Once Al was brought around to his way of thinking, they would overthrow his master and rule the city together.  It was going to be so romantic.  But it didn't work out.  About halfway through the pilot episode, Darien found himself in a sticky spot.  He had to choose between letting Alejandra get captured and certainly killed, or to rescue her and blow his own cover.  He chose to save her life, but condemned himself in the eyes of other vampires.  Desperate, he asked Al to return the favor and spare his life.  Naturally, her feelings were torn.  She had loved him, but she knew that she couldn't now trust him, what with him being a vampire and all.  Darien decided then and there that the only way he could survive was to get his soul back.  He started casting about, trying to find a reliable source of information that he could trust.  Meanwhile, Alejandra was doing the same.  This caused much concern on the part of Jack, her Watcher, but he was unable to deter her from her course.  He managed to forcibly remand her to England to be brought before the Council (see Jack's character entry to see the extent to which that didn't work out how he expected it to).

While she was gone, the Council dispatched an out-of-town Slayer (a bridge-and-tunnel Slayer from Jersey) to help keep the city, which was turning upside down at the time, under control.  A chance meeting led to this mysterious out-of-town Slayer putting Darien in touch with her equally mysterious out-of-town contact, and the pilot episode ended with Darien leaving town.

Halfway through the second episode, he was back.  With a soul.  Helping the Slayer out in a fight, even saving her life, not only convinced the good guys he was on the up and up, but made the bad guys hate him more than ever. The mystery of how exactly his soul is currently known only to him (and the head Director), but will be revealed shortly through flashbacks.