Marble Pie

Marble Pie is a female Earth pony who is a member of the Pie family. She is the youngest sister of the family. She is first seen in a flashback as a filly in The Cutie Mark Chronicles before making her debut as a grown mare in Hearthbreakers.

Depiction in the series
Marble first appears as a filly in Pinkie Pie's flashback in The Cutie Mark Chronicles. Unlike her parents, see doesn't have any dialogue. When Pinkie Pie throws her first party in order to spread cheer, Marble is shocked at first, but she slowly smiles and join in the festivities.

She appears in a picture with the rest of the Pie family in Pinkie Pride.

Marble appears as a grown mare in Hearthbreakers when the Apple family and Pie family spend Hearth's Warming Eve together. Here, is depicted as very shy and soft-spoken, often letting Pinkie speak on her behalf. She doesn't say anything other than "Mm-hmm", a characteristic similar to the equally laconic Big McIntosh, with whom she forms a friendship.

Marble is mentioned collectively by Pinkie Pie on The Gift of the Maud Pie, explaining to Rarity how Pinkie meets with each of her sisters in various locations around Equestria for gift swap meets.

She briefly appears at the beginning of Rock Solid Friendship, watching Maud Pie's graduation.

Marble appears in The Maud Couple, when Pinkie returns to the family rock farm after being unable to accept Maud's new boyfriend, Mudbriar. While Limestone is initially jealous to hear that Maud has a boyfriend, she (with a little prompting from Marble) convinces Pinkie that Maud sees something good in Mudbriar, and that he makes her happy, which is what really matters.

In the holiday special  My Little Pony Best Gift Ever, she witnesses Big Mac kiss Sugar Belle under the mistletoe, becoming saddened by the sight of it. She and Limestone later discuss with Pinkie about what gift she should get for Twilight Sparkle. They are last seen during The True Gift of Gifting, making their traditional Hearth's Warming dolls out of rocks alongside Maud, Mudbriar, and Apple Bloom.

In The Last Problem, Marble appears in Pinkie Pie's group shot during The Magic of Friendship Grows.

Trivia

 * She is the second mare in the show's history who has been involved a romance with Big Mac, following Cheerilee.
 * She is the second sister of the Pie Family to have been involved in a romance with a stallion, following Pinkie Pie, who is involved in a romance with Cheese Sandwich.
 * She is the third pony, and the only female to date, who has "Marble" somewhere in her name, following Black Marble (Season 1) and White Marble (Season 5).