Seeing I've been challenged, I'll accept. I think Robin's charm is her lack of confidence. In the bookclub, she's pretty sure of herself (
her appearance later, did she come from her room, is that Charlie's house? I'm thinking it's either than or the Petrova's place. It's ridiculously overkill setting for a bookclub... Noji clearly doesn't feel inimidated by everyone's apparent wealth... Anyway, off track), but maybe that due to being in familiar surroundings. At the college, she's polite, but she doesn't put herself forward. My guess is the gym sequence was for Sasha's benefit, and Robin got to ask her own questions as well. When people think of her poorly, she freaks out, so I kinda feel she has a low self esteem.
I think the attraction is we want to protect her. She gets bullied by Zara and Nadja into decietful acts, she gets harrassed with rumours for caring that Nika almost had a melt down, and she tries to make up for her mistakes, without expecting a free pass.
Also, she has more normal proportions, she doesn't always look like she stepped out of a magazine, when she's at the gym, she looks ready to put in the work. So I think it's also she seems a little less pretentious than some of the others - not always trying to make herself look the best.
And we probably want to rescue her from Bowser ...I mean Charlie