Princess Mi Amore Cadenza is an Alicorn and the co-ruler of the Crystal Empire together with her husband, Shining Armor. She's one of the five princesses of Equestria and is well-known for being the "Princess of Love", which is a title acquired by helping ponies overcome their problems in that particular area.
Cadence is also Flurry Heart's mother and was the foal-sister of Twilight Sparkle( Currently Cadence is her sister-in-law after marrying Twilight's brother).
Lauren Faust was involved in the creation of the story in which this princess appears for the first time. At this point of development, Cadance wasn't created as an alicorn and Faust herself was surprised to see it as such; The decision was made after she was no longer working on the series. Faust, before that, had decided that Princess Celestia and Princess Luna would be the only alicorns in the series. Like Celestia and Luna, Cadance is initially called a unicorn in the series, despite having both a unicorn horn and pegasus wings and following the term "alicorn" began to be used from the third season onwards, thus is even described as such in the last episode of the fourth season.
Not much is know of Cadence's past, but apparently, she was a pegasus before acquiring Alicorhood.
During her school times, Cadence was a popular pony and admired by many, but still was lovable with every pony around her and always willing to give a hoof to help.
Her first encounter with Shining Armor was actually an accident when she helped him to get up from the ground, which sparkled Shining's love for Cadence at the first moment when saw her.
They both fall in love with each other during the Fall Formal. Princess Cadance appears for the first time during the TV Show in the final episode of the second season of My Little Pony: Friendship is Magic. She was the foal-sitter of Twilight Sparkle when she was younger and is currently married to Twilight's brother, Canterlot's Royal Guard, Shining Armor. She was being first mentioned as "Princess Mi Amore Cadenza". She rules the Crystal Empire together with Shining Armor. According to Amy Keating Rogers, Cadance was an orphan Pegasus who was raised by Earth ponies, then Princess Celestia adopts her as his niece. She is also a close friend of Twilight Sparkle and supposedly "a very distant relative" of Princess Amore, the former unicorn ruler of the Crystal Empire.
Cadance's Cutie Mark alternates between two different ones, throughout the episode " A Canterlot Wedding". The cutie mark, a crystal heart, is sometimes represented with a single mirrored facet at its center, and sometimes with multiple facets around its center. Cadance toys use the one-facet version, embedded in a gilded decorated frame. Her cutie mark bears a strong resemblance to the crystal heart of "The Crystal Empire -Part 1 and 2". In The Crystal Empire - Part 2, when she carries the crystal heart, her cutie mark shines as if surrounded by its magic.