Well, the MC and Sandra have at least had sex. I don't think I'd call what they have "chemistry" either, but it's clear that Sandra has some sort of feelings for, uh, Muffin. The MC has given the impression that he ended up roped into a relationship he never really wanted from the very start, but then again he has that flat emotional aspect that's become somewhat of a signature of this particular writer's male MCs and so it's hard to really know what he's thinking. That said, it's possible to play the story in such a way that he's kind and somewhat sweet to Sandra, which Haley never was to her boyfriend. Still, it feels — I'm not going to go through the script and check — like the majority of their time together is arranged by someone else. Mostly Haley. Which is the long way 'round to agreeing that she's a plot device meant to delay the moment the MC and Haley are forced to confront their relationship. What remains to be seen is whether or not staying with Sandra too long affects the outcome of that confrontation.
That and just how badly anyone who stuck with Sandra is going to feel when they break up. I've said it before, but on the "stay with her for now" route I feel like I'm about to kick a puppy that's done nothing to deserve it.
Watching the MC drift through encounters and experiences without seeming to do much of anything to instigate them (Lisa and the changing room with Sandra are the only proactive moments I can remember), I find it helpful to remind myself that he isn't the titular character. We're playing the main character, but he's not actually the main character.