Keynote is based on a slice of my childhood. Her cutie mark is a barrel key (not skeleton, they're different okay?) with a slip of parchment wrapped around. Paper can be rolled inside barrel keys to pass information or hide secrets, a method I used often as a child. Her cutie mark indicates trustworthiness as a secret keeper. The overall shape of the key is inspired by Pottermore's Gringotts key. Her magical talent is locking and unlocking objects. I'd also like her to be able to affect other unicorns' magic by casting spells to visualize the magic patterns and unravel them, but this skill would come later in life after much study.
Keynote has two older brothers (my brothers' ponies - a pegasus named Tintype and an earth pony named Adam) and a pet named Clāvis (Latin for "key") who carries messages during her travels around Equestria. I'm still working out Clāvis' design.
* Please do not copy, steal, or alter elements of my pony to make her your own. That's not cool.
edit 8/4/13 Simplified cutie mark and changed eyelashes. Added shading.