Rapid is a new character introduced in Metal Slug Attack.
A Rebel Army officer in charge of their supply unit, Rapid is also the best train conductor in the Rebel Army, as no one is able to best her in steering trains. Her map route and schedules are also known for being impeccably accurate.
Her MSA description states that while her military career is successful, her personal life is a mess due to a large number of gossips and secret admirers. If she is interested in someone, she will ask them if they would like to board her train.
First appearing in the Extra Ops "Supply Lines", she's informed by a soldier that Abigail has ordered her into battle to disrupt the Regular Army's supply lines so she can break their defences as she is the best candidate for the job. Skeptical because she has never fought and that trains aren't supposed to be used as weapons, she follows them anyways. Learning about their supply routes from the soldier, she spots Vita flying about and manages to convince her to help with the mission.
After Vita shoots down the new Regular Army Helicopter, Rapid is surprised and learns that Vita likes to distinguish herself in battle. She tells her that she prefers moderation instead, and offers Vita a ride back to base. Several soldiers also want to board the train, but she tells them to try again next time, much to their disappointment. Back at the Rebel base, Rapid submits several compensation bills to Abigail to cover the cost of the mission.