I like the idea of giving Rarity some of Rainbow Dash’s traits, a cocky fashion diva who is extremely boastful and agressive. But also very cunning and has great persuasion skills. Her ambition knows no bounds. Wanting to prove she can rise from her lowly origins to become a world class fashion designer.
She also has a sisterly bond with Fluttershy kind of like Sonic and Tails. Rarity can often be overbearing with Fluttershy, sometimes even making her uncomfortable. But despite Fluttershy truly is grateful that Rarity never gave up on her when others did.
Rarity’s idea of generosity is giving others the ability to seek out their dreams. This is why doesn’t try tell Spike to move on from Rarity.
Rarity is the rags to riches story. She is a seductive lady with a dark past. She is more afraid of dirt than blood (I’ll try not to make it too edgy). She is somewhat of an antiheroine.
She is the most ambitious of the group and in many ways motivates them to never give up. She has such will that inspires others. She believes anyone has the potential to be great. Even Fluttershy. For Rarity, fashion is a way of transforming a lowly pony to become higher than what she was.
Rarity often has a conflict between wanting trying to look good in front of others and doing what she truly wants. Over time as she gains more confidence, she starts to be more bold in her fashions not as worried as following the trends completely.
Basically she is an elegant lady with a rough side to her.