Rainbow Dash

The main character of the series, Rainbow Dash is a cyan blue pegasus with a rainbow-coloured mane and tail.

Love for Applejack
"'The fact is, Rainbow ran on something else, a sort of fuel that transcended normal thoughts and feelings. [...] You see, crushes don't last forever. [...] But Rainbow Dash was different. She grew into it. And, with time, she would find that such a desire, such a hope, such an amorous obsession would be the one thing keeping her ticking.' --Narrator"Rainbow Dash's relationship with Applejack starts when she crashes into the land of Sweet Apple Acres while moving into Ponyville. She finds Applejack very beautiful and instantly develops a small crush on her. Later, just like in canon, they had a rocky start to their friendship, where Rainbow Dash was annoyed by Applejack's constant nagging about her brashness and even did mean things on purpose to try to rile her up.

However, after having outright butted heads with Applejack during the Iron Pony Competition (Running of the Leaves), Rainbow Dash found herself wondering how Applejack could stay stable while she herself felt terrible. A talk with her made her better appreciate Applejack's philosophies in life and also appreciate her as a friend. Then, in the following months, her crush on Applejack grew from "a tiny flutter" to "a full-on friggin’ tidal wave", and she has been keeping this crush a secret. 

Rainbow Dash's journey, while seemingly disconnected from the romance for the most part of the story, is revolved around her love for Applejack.

Medical Condition
Unlike canon, Rainbow Dash suffers from Pegasopalitine Atrophy, Level Three, a rare degenerative muscle disease that degrades the muscle tissue in the limbs of pegasi and is fatal when it eventually attacks the immune system of its host. Throughout the story, Rainbow Dash refers to the sensations in her limbs as "noodly", "tingly" and "numb", relying on pills that temporarily stave off the effects of this disease. This illness serves as an explanation as to why Rainbow Dash is often seen hovering in canon instead of staying put on the ground.

This disease is genetic and was passed down to Rainbow Dash from her father, who died of the disease when she was a teenager in flight school.

Knowing of her condition, Rainbow Dash trained herself physically very hard, especially her wings which wouldn't be affected, which is how she managed to maintain her fitness longer than her dad did. However, the illness caught up to her and she found herself having to rely more and more on the pills prescribed by Nurse Redheart to stay on her hooves. Nurse Redheart proposes a permanent solution to her problem- a surgery that will transfer the energy from Rainbow Dash's wings to her limbs which will allow her to survive, but take away her flying abilities. Flight being a vital part in her life, Rainbow Dash refuses to accept this proposal.

As such, her condition only continues to worsen, especially when she starts going on physically-demanding missions, as seen when she collapses and has to take the pills with increasing frequency to the point where Nurse Redheart bars her from collecting further pills. Luckily for Rainbow Dash, she runs into Zecora who makes a magical brew that achieves the same effect as the pills. She mentions that the brew may cause hallucinative side effects, but aside from the first ingestion of potion, Rainbow Dash has not noticeably suffered such side effects.

Trivia

 * As in all of ShortSkirtsAndExplosions' stories involving Rainbow Dash, Rainbow Dash is often described as "petite" and this has become a running gag.
 * Rainbow Dash's association with the moon and Princess Luna is a reference to her exploits in a separate series by the same author, Austraeoh.