Vapor Trail is a female Pegasus pony and Wonderbolts cadet who appears in Top Bolt and Parental Glideance alongside her best friend Sky Stinger.
Backstory
In her time of Stratusburg she lived in the cloud and she gets a lot of attention but Vapor Trail doesn't like attention her backstory begins with attention.
Her parents loved her and wanted to keep her safe because she was the only filly in the family. Her parents gave her a lot of attention, and she didn't like it one bit. Until she met Sky Stinger, she was very lonely in the house besides her parents.
Side Note
We have very little bit of information about her information because on the MLP wikia it doesn't tell us much about her.
Personality
Vapor Trail is a soft-spoken young Pegasus who starts her Wonderbolts trial week helping her best friend, Sky Stinger, perform his tricks...at the cost of her own flying ability. She learns that being a Wonderbolt really is important to her, and dedicates herself to improving her own flying, earning a spot at the Academy.
She also likes helping out her friends the much she can, and she is kinda clumsy.
Depiction in the series
Vapor Trail first appears in the season six episode Top Bolt as an applicant to the Wonderbolt Academy. She is introduced as Sky Stinger's timid and soft-spoken best friend and wingpony. During the classroom portion of applicant testing, Vapor praises Sky Stinger's flying skills to instructor Rainbow Dash. However, it is revealed during freestyle training that Vapor Trail supports Sky when he flies by blowing wind under his wings. As a result, she pays no attention to improving her own flying.
In the cadet locker rooms, Vapor Trail explains to Twilight Sparkle and Rainbow Dash that Sky Stinger grew up with a lot of siblings and often competed with them for their parents' attention while she grew up as an only child with parents who smothered her with affection. When Sky struggled to impress his parents with his flying, Vapor gave him a wind boost and congratulated him for performing a flipping loop. Ever since, the two have always flown together, with Sky unaware of Vapor helping him and Vapor not realizing that her help was actually a detriment to his flying development.
Despite this, Vapor Trail wants to see Sky Stinger realize his dream of becoming a Wonderbolt, not concerned with becoming one herself. Twilight and Rainbow Dash help her achieve this by help both her and Sky improve their flying. However, when Vapor eventually reveals to Sky that she's been supporting him and that he isn't as talented a flyer as he believes, Sky lashes out at her in anger, and the two have a falling-out.
Rainbow Dash later approaches Vapor Trail to compliment her flying, and Vapor comes to realize that being a Wonderbolt is indeed something she wants for herself. With Twilight and Rainbow's guidance, Vapor makes amends with Sky Stinger, and they assist each other in improving their flying skills. In the end, both Vapor and Sky impress the testing instructors enough to be granted entry into the Wonderbolt Academy.
In the season seven episode Parental Glideance, Vapor Trail makes a cameo appearance walking alongside Sky Stinger at the Wonderbolt Academy.