Rose Leader X-wing was a heavily modified version of the X-wing starfighter used by Rebel commander Ruby Tachibana of the Alliance Starfighter Corps and was part of Rose Squadron. It was often commanded by only Zwei, Ruby's trusty sidekick astromech R2 unit that looked after the starfighter when docked.
Before it became known as Rose Leader X-wing, it was known as Red 7 during the Death Star Assault orbiting the gas giant Yavin. Being one of the five surviving starfighters in the attack, most of Red Squadron were killed and instead Ruby then proposed that she and Weiss go on to form Rose Squadron together.