Mel

"Mel! Over here!" — Virgil

Mel is the titular main protagonist of Portal Stories: Mel. Being an Olympic runner, she arrives to Aperture: Science Innovators to participate in experiments as a test subject. After oversleeping in a faulty relaxation chamber, Mel travels with Virgil around now lifeless building to defeat AEGIS and ultimately escape to the surface.

Test Subject
At the beginning of the game, Mel arrives at Aperture Science’s constructed city, guided along her way by recordings of Cave Johnson. She follows his instructions to an Aperture Short-Term Relaxation Vault, where she is supposed to sleep for only a few minutes. She is woken up an unknown amount of years later by a voice positing itself as Cave Johnson, in a ruined facility.

Meeting Virgil
Guided by “Cave Johnson”, Mel picks up an early version of a Portal Gun, and then follows his instructions to “keep testing another shaft”. However, as it becomes more obvious that it was not an earthquake that destroyed the facility, the voice gives up and reveals himself as Virgil, a maintenance core. With his help, Mel ascends through Old Aperture and rescues Virgil, who has fallen off his rail.

Escaping Old Aperture
With Virgil rescued, he and Mel have to face the sudden flooding of Old Aperture by an unknown, all-powerful assailant. They manage to escape and enter the newer facility above the salt mines, while Virgil attempts to guide Mel through increasingly dangerous testing tracks. The assailant- which is revealed as the Aperture Employee Guardian and Intrusion System, or AEGIS- takes over many of these testing tracks in more attempts to kill the pair. Virgil plans to hide for a while in the overgrown tracks GLaDOS made, to confuse its scanners.

Organic Complications
With Virgil’s plan working, Mel continues to go through the ruined test tracks, eventually leading to an office where they can access the security systems. Virgil attempts to hack in- with Mel eventually activating the admin override. Together, they agree to take down AEGIS by manufacturing friendly turrets and training them to attack its memory banks.

Confronting AEGIS
When approaching AEGIS, Virgil cannot accompany Mel, as there are no management rails. Inside of AEGIS’ offices, water that Virgil sent up earlier has destroyed a quarter of the server blocks, and Mel must destroy the rest with the previously created friendly turrets. Once done, she crawls into AEGIS’ mainframe and shuts it down from the inside, but not before revealing that AEGIS’ primary target was GLaDOS herself, after she killed all the scientists. Regardless, AEGIS is destroyed and Mel returns to Virgil at an elevator shaft.

Saying Goodbye
Virgil calls an elevator to set Mel free, and they share a small moment of friendship before she leaves, watching him go. When the elevator reaches the surface, the destroyed, heavily overgrown Aperture is revealed, with Mel looking in shock at what the world has become in her absence. Her fate beyond this is unknown.

Trivia

 * Mel is a removed character from Portal 2. Originally planned as its main campaign protagonist, she was replaced by Chell and reserved for multiplayer mode where two would cooperate, before both were replaced by robots. It should be noted that Mel's model in Portal Stories: Mel is totally different from her concept art for Portal 2.
 * Apparently, Mel was a competitor in the 1936 Nuremberg Olympics. Virgil states that “she deserved that silver medal”, implying that she ranked bronze in her sport.