Flash Sentry and Lightning Dust are childhood best friends and really, each other’s only friends for a very long time. Both grew up in military families so they constantly had to move every time their parents got stationed in another town, which meant that making & keeping friends was very hard. Luckily, their folks are in the same unit so they were never truly alone. They kept each other company and explored every inch of wherever they went together – having adventures, racing, learning how to fly, etc.
 
Flash made friends wherever they went while Lightning preferred to keep to herself (and Flash) because she didn’t see the point of making friends just to lose them and be sad later. Flash never went anywhere without her so she played nice and learned to like impressing them with her skills, which made things very fun and daring. He’s had to patch her up more than a few times.
 
As they got older, she was his calm hoof + his greatest ally and he was her protector + safe space. He was always by her side when she called and she gave him advice on mares – they challenged each other and knew when to call each other out (Flash for his temper and Lightning for her brash and careless showboating) and they really were their own pony together – stronger than the sum of their parts. Although Lightning was angry when he decided to enlist in the guard and they didn’t talk for a while they eventually made up and train together for their respective branches of service and she teases him about his major crush on Princess Twilight.
 
He’s the only pony allowed to see her cry and here he is comforting her after her dad died – he was a guard as well – first Pegasus Captain of the Royal Guard – and died protecting ponies. She was secluded and miserable for a while and Flash knew to give her her space to grieve but finally he visited and was the friend she needed. Flash thinks there’s nearly nothing that tops how beautiful her genuine smiles are.