Ooh, what a cute little topic! I'll put my story in here to contribute!
My OC, Silver Herald, is generally a more reserved and shy kind of stallion. He doesn't open up to other ponies often, but when he does, he tries to be the best possible friend he can be.
As a colt, his little sister Feathersong was his best friend by far. Since she was only a year younger, they could play together and mess around without getting the whole "ewww, I hate my sister!" sort of dynamic going. Her more outgoing and confident attitude brought out quite a bit of the stallion- it's like he was a different pony when he was around her. Never shy, never stammering, never afraid to mess around with his playful younger sibling.
One night, as the two siblings were trying to get some sleep, a strange pony broke into the house and tried to kidnap Feather. She cried out for help, which awoke the young Silver, who looked out the window to see a masked pony running off with his little sister. He yelled at the stranger, and made the split second decision to jump out of the window and topple the kidnapper, sending all three ponies tumbling to the ground. All of the commotion caused other ponies around the neighborhood to wake up and look our their windows to see what all the fuss was about. With the eyes of the whole community on him, the kidnapper ran away empty-hooved.
From that moment on, the Silver stallion would bear the cutie mark of a blue heart, symbolizing the care and affection he had for others. He's a rare kind of pony that prefers to put others before himself, a quality that embodies itself within his rather simple looking cutie mark.
Here's a picture of the two, for reference!