Diamond Tiara is the daughter of Filthy Rich and Spoiled Rich (nee Milk). A mean girl type pony, she and her friend Silver Spoon would often make fun of the Cutie Mark Crusaders, calling them Blank Flanks.
She is very manipulative, as seen in "Family Appreciation Day," deliberately making Apple Bloom insecure and embarrassed of her Granny Smith, even though she'd never felt embarrassed by her before. By convincing Apple Bloom that Granny Smith was kooky and weird, she bullied Apple Bloom into believing that she should be ashamed of her family.
In "Ponyville Confidential" she was appointed editor-in-chief of the school paper, the Foal-Free Press, and ordered the Cutie Mark Crusaders to write a gossip column. The Crusaders felt bad about spying on other ponies and exposing their secrets, but Diamond Tiara put more pressure on them, becoming drunk on power. By the end of the episode, Ms. Cheerilee admitted that she obviously gave Diamond Tiara too much power, and stripped her of her editing responsibilities, reducing her to printer operator.
In "Crusaders of the Lost Mark," when running for Student Pony President against Pip Squeak, she became very controlling and abusive of Sliver Spoon, causing her to abandon Diamond Tiara as a friend and costing herself the election. Her mother, Spoiled Rich, scolded her daughter and it is only then we learn that Diamond Tiara's bad attitude comes from her mother. The Cutie Mark Crusaders take pity on her and help her become a better pony.
Call of the Cutie
Family Appreciation Day
Crusaders of the Lost Mark