Possible spoilers: To hell with the Duckie vs. Blane question. Steff's the only interesting one.
1. He's fearless: Duckie's afraid to tell Andie he loves her, Blane's afraid of his friends, but Steff walks up and hits on her right in front of his friends without caring what they'll think. He exploits Blane's shame about Andie but clearly feels no shame himself.
2. Duckie's suffering puppy love, Blane's just curious, but Steff has been after Andie for years. When he says it's "not *only* for sex," he's probably telling the truth, in his own sarcastic way. Key scene, when Blane and Andie walk in on Steff and Benny Bimbo, and Steff clearly wishes he were with the hot geeky girl, instead of the vapid blonde, even if Benny is better-looking, sexually available, and approved by his friends. And it's not only the challenge attracts him to Andie: despite his arrogance, he's probably been turned down before. He just suspects that with Andie, he'd have something real, and with Benny, whether her ecstasy is real or faked, it will be conducted according to "Cosmo."
3. He's willing to fight for what he wants, i.e., sabotaging Andie's relationship with Blane. OK, he's fighting dirty--but he's fighting at all. The others are about as decisive as Hamlet.
Right, enough deep thought devoted to what should probably be regarded as another teen movie. I give full credit to Spader, who fleshed out what could have been a one-dimensional baddie by making him really interested in Andie in his own sociopathic way, instead giving lip-service to a script that could have been played with the character being vicious solely out of hurt pride and snobbery.
