She's out to play him, to use him somehow against Angel. He knows. Doesn't care. She's hot; he's straight and male. Fact is, he's not ready for a relationship not based on deceit.
After a while, he realizes she's not evil. Morally ambiguous, maybe, but he has a ton of experience there. She's more frightened than anything else, a creation of the Senior Partners who serves them mainly because she doesn't know what else to do. Xander comes to find a vulnerability in Eve which is endearing. He wants to protect her.
He swears he won't lose her, too.
Eve and Xander belong to Joss Whedon and Mutant Enemy.