Diamond Tiara is a snobby, conceited school-age Earth pony and a minor antagonist who first appears in Call of the Cutie. She and Silver Spoon are classmates of the Cutie Mark Crusaders. She bullies the Cutie Mark Crusaders, along with her friend Silver Spoon, and calls and picks on other ponies without cutie marks, "Blank Flanks". She is also the daughter of Filthy Rich, a successfull businessman who is friends with Granny Smith and the owner of his own company, Rich's Barnyard Bargains. Due to her cruel behavior, she is usally punished by him, for example in the ending of the "MLP: FIM" episode "Family Appreciation Day", being forced to wear a pair of bunny ears and hop around watering cans while singing the Alphabet song.
In Call of the Cutie, Diamond Tiara and Silver Spoon invite Apple Bloom and Twist to their cute-ceañera, a party for those who have recently earned their cutie marks. (The name is a pun on the Spanish quinceanera, a girl's "sweet 16" or coming-of-age party.) It's heavily clear that they were only invited so that other party attendees can mock them for their lack of marks. Twist manages to gain her cutie mark after finding her talent (making candy), but unfortunately, Apple Bloom doesn't earn hers before the party.
At the party, Apple Bloom tries to hide her lack of cutie mark with a table cloth, but bumps into Diamond Tiara and Silver Spoon. She claims that she got her cutie mark earlier, but doesn't want to show off and take attention away from the party's guests of honor. Diamond Tiara replies that she didn't want to see Bloom's cutie mark anyway. As Apple Bloom prepares to leave, she trips over the long table cloth and hits her head on the table that a radio was stationed on, thus exposing her flank. Diamond Tiara and Silver Spoon see that it's blank and don't hesitate to make fun of Apple Bloom, who is soon joined by two other blank flanks, Scootaloo and Sweetie Belle. The teasing gets ignored as Twilight Sparkle and Applejack explain that being a blank flank means still having many possibilities open. Diamond Tiara is later seen standing alongside Silver Spoon in the stairway, watching the other ponies play party games.
She appears again along with Silver Spoon on stage at the talent show in The Show Stoppers, as well as during the lesson in The Return of Harmony Part 1 and in The Cutie Pox with Diamond Tiara trying to prove in the school yard that Apple Bloom's cutie mark is a fake. Later she asks Cheerilee if it's really fake and is seen being jealous of Apple Bloom's loopty-hooping ability.
She appears in Family Appreciation Day, alongside her father, Filthy Rich. When she visits the farm with her father, she points out Granny Smith's odd behavior to Apple Bloom and convinces her of how humiliating it would be if Granny Smith spoke to her class. After Granny Smith impresses Apple Bloom's class with her presentation of how she helped found Ponyville when she discovered and learned how to cultivate zap apples, Diamond Tiara openly denigrates her. At the end of the episode, her father, evidently mad that his daughter insulted his most valuable produce supplier for his business, forces her to participate in singing to the water cans, much to her
frustration. In Hearts and Hooves Day is given a card from Alula to her surprise and then is seen checking her hooves and then reacts to Cheerilee.
In Hearts and Hooves Day, she is given a card from Alula, to her surprise.
In her next appearance in Ponyville Confidential, she is the chief in charge of the school paper: the Foal Free Press. She appoints the Cutie Mark Crusaders the writers of the gossip column to try to revived Diamond Tiara. When the Cutie Mark Crusaders attempt to resign their position, Diamond Tiara blackmails them with embarrassing photos. Diamond Tiara is later stripped of her position by Cheerilee, who gives it to Featherweight. In the end of the episode she has been demoted to take care of the printing press and the previous holder of that position, Shady Daze becomes the photographer and decides to take a picture of the demoted Diamond Tiara. The flash from the camera dazes her and makes her stumble upon the printing press, making her dirty with ink. As everyone is laughing at Diamond Tiara, the episode closes with her looking bitterly at the audience.
In One Bad Apple, she and Silver Spoon mock the Cutie Mark Crusaders float for the Summer Harvest Parade. When they notice Babs Seed, they point out her blank flank and ask her if she will join the CMC. Babs responds by the calling the CMC "crybabies" and destroying their float, to Diamond Tiara and Silver Spoon's delight. Babs joins them in bullying the CMC, until they save her from a sabotaged float. As Babs is about to leave Ponyville, Diamond Tiara bemoans her and Silver Spoon being left with the "blank flanks", causing Babs to turn on them, threatening to tell their mothers about their bad attitudes. In fear, Diamond Tiara and Silver Spoon back up and fall into a mud puddle, with Diamond Tiara's tiara ending up on the head of a pig.
In Flight to the Finish, she and Silver Spoon compete for the honor of being the flag bearers of the Equestria Games. They spy on the Cutie Mark Crusader's rehearsal, and realize that the trio might actually win. They then decide to undermine Scootaloo's confidence by mocking her inability to fly. In the end, Scootaloo and her fellow Cutie Mark Crusaders perform their act, winning the right to be flag bearers, to Diamond Tiara's displeasure.
In Twilight Time, Diamond Tiara and Silver Spoon learn of the Crusaders' "Twilight Time", their personal learning sessions with Princess Twilight, and ask to come along. After this initial encounter, Diamond and Silver tell the rest of their classmates about Twilight Time and pressure the Crusaders in letting them spend time with Twilight as well. When the Crusaders later try to exclude Diamond and the others from Twilight Time, Diamond and Silver lead the other foals to the Golden Oak Library against their wishes.
When the Crusaders are unable to perform the tasks they'd been learning during Twilight Time, Diamond mocks them, saying they aren't Twilight's "entourage" any more. At this, Twilight remarks that none of the other foals came to Twilight Time to learn, and Diamond leaves the library in disappointment among her classmates.
She makes a brief appearance alongside Filthy Rich in Inspiration Manifestation.
She and Silver Spoon make a cameo appearance along with various ponies in the end of Twilight's Kingdom - Part 2 to see the newly formed Friendship Rainbow Kingdom castle.
Diamond Tiara appears along side Silver Spoon in Bloom & Gloom during Apple Bloom's first dream. The pair of bullies this time tease Apple Bloom for having the "lamest cutie mark ever." Later in the episode, Diamond Tiara along with other townfolk start running from a Twittermite infestation that's destroying Ponyville.
Diamond shows a significant change of heart in Crusaders of the Lost Mark. She begins the episode by running against Pipsqueak for class president, but when she loses, she is berated by her mother Spoiled Rich. After the Crusaders spy on her soliloquizing, they invite her to her clubhouse. Upon hearing Pipsqueak reveal that there is not enough money in the budget to repair the school playground, she runs off to the school seemingly to call out Pipsqueak and reclaim her place as class president. When Spoiled Rich berates her again, Diamond Tiara stands up to her mother and asks her to give a note to her father. Her father provides enough money to repair the school playground, and she helps guide the other foals in fixing it.
- Diamond Tiara along with Silver Spoon are the only two villains to be seen in all 5 seasons.
- They are also the most recurring villains in the show.
- Ironically, Diamond Tiara's voice actress Chantal Strand has previously voiced two notable characters who are the polar opposite: Cassie on Dragon Tales and Bijou on Hamtaro.
- Diamond Tiara will become the bully of Winnie the Pooh in Pooh's Adventures of My Little Pony Friendship is Magic.
- Diamond Tiara will become a bully but also an nuisance, of the FT Squad in The FT Squad's Adventures in My Little Pony: Friendship is Magic.
- Diamond Tiara along with Silver Spoon will become Emerl's enemy in Team Robot In Back To The Future & Team Robot In Back To The Future Part II, but becomes Emerl's friend in Team Robot In Back To The Future Part III.
- Diamond Tiara along with Silver Spoon are also best friends with Arry and Bert and Ripslinger.
- While during Filthy Rich's speech to Cheerilee's class in his debut episode, Diamond Tiara is alert and attentive during his speech, while most of the other students are all bored and are all taking naps. It might be implied that she highly respects her father.
- Also, in that same episode, Diamond Tiara suffering punishments by Filthy Rich somewhat looks exactly simillar to Cera suffering horrible punishments and forbidden to do things by her own, strict and cruel father, Topsy, from "The Land Before Time", a film series known for having musical numbers.