And I do think this relationship brought some needed color to Uno-san. Hopefully, the author will add more depth to her character; as much as I like what I've seen so far, she's still paper thin to me (mainly a person on whom the MC is fixated).
I assume this is in reference to Satou-san, not Uno-san, judging by your last sentence?
In which case, I would've agreed prior to chapter 3, but I think chapter 3 told us quite a lot about her in very little time. We get to know her family circumstances, her apparent dedication to raising her sister, and presumably as a result, her not wanting to invite the idea of "falling in love" into her life. We see that she puts on a bit of a facade at work, same as Uno-san. We see that she's surprisingly interested in Uno-san as well, and is unusually thoughtful toward her (relative to how she normally is, according to Subaru). We see that she probably has some emotional issues to work through, given how some of the same signs of depression that are present in Uno-san (lack of motivation to clean her room) are present in Satou-san as well.
She hasn't had a ton of focus thus far, and I'd like to see her character be further fleshed out, but I don't think it's fair to call her paper thin, either.
I agree, she was a tpical laconic character before this chapter but some things changed with chap 3.
I feel like her family situation has deep meaning on the way she acts, and the fact that imotou was not surprised by Uno-san being a woman tells me that maybe, satou has already adressed it with her, maybe with her late parents too.