Recklessness is a trait that Steven actively avoids. He's a perfectionist, he weighs all of his options before moving forward. He's brave, yes, but not in the straightfoward way that characterizes Gryffindors.
Hardworking and extremely patient, Steven would have settled in Hufflepuff but I don't think he would have excelled -- Hufflepuffs aren't inherently competitive and Steven has a competitive drive to be the best that's best fueled by rivalry.
Though intelligent and studious, most of his studies are devoted to the pursuit of being a better agent -- his skills are varied but all of them have to do with furthering his career. He's got a single-minded interest in being the best of the best, even if he doesn't seek recognition or fame. He'd fit in well with Ravenclaw, but other qualities are a priority.
Steven best fits in Slytherin. He does best when presented with rivalry and competition, when he feels like he needs to push forward rather than settle and be complacent. Sometimes, that competition is himself. He's got a strong drive to do better, be better, gain more respect and power within his circle -- he doesn't seek active command in any way, but he has a strong drive to be exceptional and be invaluable to other people. He must see missions through to the end, he'll do that by almost any means necessary, and he seeks unconventional, underhanded methods of solving problems while seeming invisible. He can't stand being outplayed, and he believes in being the best in all ways rather than relying on a partner or team.
The Scarf of Sexual Preference says: No Thanks, Maybe Later.