Steven is in-the-closet gay, so probably some dude we haven't met yet. Though if we asked Steven, I bet he'd say the MC.
Seriously, it seems pretty obvious at this point what Steven's secret is (if I'm right, dev, please don't change it: the groundwork was laid really well!). He likes looking at the MC's cock, likes pulling his own out in front of other dudes, and for all his bluster about getting laid still leaps at any and every opportunity to go wherever the guys (specifically the MC) go regardless of what chances he may be leaving behind - and generally attempts to go in some state of undress. Let us consider the dildo selfies, the sincere attempt to "finish each other off" after Saeki leaves the bathroom, and his excitement to run naked with his bestie. Throw in his "prank" with the sleeping Rene (sticking his dick in his face and fantasizing about getting oral sex from him) and how he blatantly checks out the MC's dick when walking in on him pissing, and I think we can all do the math at this point.
Yeah, he hires hookers. Notice he only does that WITH or FOR the MC, though. There is not one single instance where Steven has gone off on his own, and if we say "well, he hasn't had time!" there's also not a single story of Steven going off on his own. The closest thing to it is when he goes out with his dearest bff for blowjobs to win a bet. And they're still in the same building on the same mission when that goes down. I wonder who he thought of to get himself erect, hmmmm?
Whenever you're ready, Steven, come on out of that closet. We'll welcome you as the best "MC's Best Friend" character ever, since you pose no threat to steal any potential LI. We may not be right for you (or hell, maybe it's an upcoming route some people might really like, what do I know?), but you'll always be our friend. Please keep paying for our hookers, though.