I think it's pretty clear that Ian represents the devil on the player's shoulder, with Hana (etc.) playing the angel. I think people telling themselves Ian is a good person are fooling themselves - he reveals early on that he chases other women in order to deliberately torment Mina, which makes her a more enthusiastic lay.
But the point, narratively, is that this game is attempting to serve two audiences: those who want to unleash their inner devil when they play, and those (with respect) who have moral qualms about playing a male dom game and want to tell themselves that they're not bad people for partaking in what is objectively a horribly immoral exercise (making money from and assisting with the degradation of vulnerable women).
Ian serves a useful role by giving the player someone to emulate and become close to, if you're indulging the devil on your shoulder, and someone to resist and become adversarial to if you're indulging the angel on your shoulder (which is why in the latter route you steal his girlfriend).
P.S. Most games serve two audiences. See for a close analogy Estate: Dominate, where you can be a decent person who does his best to ensure that the other victims of the Sterns are protected and treated well, or you can be someone who's morally indistinguishable from your adversaries, playing the same games with both them and with their other victims whenever your available leverage gives you the opportunity. Neither is wrong, it's just about where your head is at in terms of what you enjoy.
P.P.S. When I was younger I was more conflicted about what I enjoy, but now that I've realized that as long as I treat real people with love, I can approach fictional situations with maximum heartlessness and savagery, I eschew the "I'm just a nice guy who's basically forced, forced, I tell you, to participate in this thing that secretly thrills me" paths that these games offer.