Honestly, can you come up with a more boring character than Stan? He has neither looks nor personality; he is simply devoid of anything interesting. If he were the protagonist of the game, he would be cucked by every breathing female organism in the universe. He isn't the daring creep who got the guts to blackmail Lena, which I believe would make his character far more intriguing. However, he is portrayed as a nice creep, which in my opinion, is the worst decision Eva made. His interaction with Lena feels so forced, especially when she accepts his feelings. I mean... what? What could Lena possibly find appealing in him? I can only imagine she would entertain his affections out of sheer pity.
I don't know that I'd call Stan boring, but that's probably because I've known a few guys IRL who are very like Stan in a broad sense: deeply insecure, emotionally very needy, and trapped inside their own head when it comes to women and what relationships are and how they work. Someone who is interested in the
idea of woman X in their heads, not in dating the living, breathing woman. These guys are complex individuals. They have trouble dealing with the facts of who they are and they retreat into their own fantasies about themselves, so it's easy for them to also retreat into fantasies about other people - women, men, friends, family, coworkers, neighbors, etc. - instead of having actual relationships that take work and attention and have ups and downs.
But I agree with you that Stan wouldn't be a good protagonist. The guys I've known who he reminds me of are often miserable people to be around. They know they don't connect well with others, they want to change, but rarely do anything about it, preferring self-pity and grand fantasies to realizing they have the authority to make personal choices that would start to improve themselves and their life.
However, I disagree that Stan being a "nice creep" was a bad decision by Eva. He can be helpful to Lena if she needs a photographer and doesn't have other options, or does but doesn't want to use them. He can be helpful if she wants to indulge her exhibitionism fetish. You're right that Lena might well be nice to him out of pity; in one of my saves, that's exactly what she does, and I think that's defensible. I think it's great that Eva has the situation where, if you are nice to Stan right along, on your walk in the park with him you can encourage him to have self-confidence, or learn to socialize better, or to get in shape - all things Stan and the guys who he reminds me of need encouragement to do. It's after that option where Lena could kiss Stan, or tease him, or more realistically, tell him to be patient or that he needs to work on social opportunities. I agree with you that it seems far-fetched, in my opinion, for Lena to see Stan as a possible romantic partner, but I think it's totally fine of Eva to give players that option if they want to use it.