I mean, it is a porn game, not gonna find very complex characters in any or almost any of them. But in the case of SoB I think they are well designed as far as it opens different types of kinks or interests.
Elijah: more of a learning as he goes, "so it's fine to do this, let's try this then" kind of a bit innocent and so on, and adds the fantasy side through D&D and exhibitionist side through the streams and the cosplay side.
Granpa: a bit slower, more on the exhibitionist and slow corruption with some teasing and so on plus those interested in the maid kink. I do think he's Mr X so add that to it here.
Dean: here we have a classical BDSM story with a character that's not only "gonna tie you and fuck your ass as hello" but in a more interesting and caring portrayal of dom/sub relations.
Dean secretary: here we have both the BDSM but also the lines for those interested in a more confrontational relationship and also the parts on prostitution.
And so on. So I don't think it's about creating a dozen very believable or complex characters. To be honest, maybe it's the sociologist in me speaking, but people are generally not that complex in real life either. So in this case, I think he did create a diverse cast of characters that allow you to actually see different sides of Riley (even acting as Rebecca, another side, as a "dumb bimbo") and different kinds of stories and events to play with so it doesn't get repetitive. Because in a lot of these games, what you end up is having a girl hunted by a set of guys/gals all of them alpha and all of them more or less acting in similar ways. It gets stale after some time.