Princess Cadence

Princess Cadence is a main character of MLP: A Royal Pain.

Princess Celestia
Celestia and Cadence have some sort of rivalry. In Cake Monster - Part 1, when Cadence called Celestia fat, she responded with a faint "oh...", but then Cadence told her to get lost, and she punched Cadence and told her to get out of her hood. She then bragged on how beautiful she was and warned her that she could fire her from protecting the Crystal Empire, and make Shining Armor the ruler.

Shining Armor
Princess Cadence and Shining Armor are married. In Cake Monster - Part 2, Princess Cadence gets jealous of Princess Celestia because Shining Armor tied her up with a rope, but it looked like a dress to her.