To put it simply, Rarity takes on the Fashionista role in this all girl group, only rather than just being that girl and going to to the mall all the time and such, she just really likes fashion and making it herself as well. She even says at one point in the EG movies that creating clothes for her friends is what she lives for because she genuinely really wants to make her friends and, well anyone really, to stand out and be comfortable in how they look.

Applejack, who is notoriously not a fashion type person and is the hardworking gal who grew up on a farm, ends up enjoying what Rarity creates for her because Rarity knows what AJ would like!

I think Roberta/Lapis could work in this, tbh, as Rarity would probably like to see what she can do to help make the poor girl happy, even if its something as small as giving her an accessory for her hair or some such. Like I said in the thing above, Rarity embodies the Element of Generosity, so she's going to find ways to help!

eeeeeven if she might be a little overbearing at times lmao