(Capcom's primary storyline)
Mylène Beardsley was the head of the prestigious Beardsley family despite her young age, being the seventeenth head. She lived in a castle with her butler Gilliam. She was later revealed to be the byproduct of genetic experimentation, as well as working on an enhanced version of the T-Virus, the Neo-T Virus. She was also responsible for the creation of the Bio Organic Weapon Millenia.
Her father subjected Mylène to experimentation allowing her for full access to her brain. This later ended up augmented further by her with modern cyber techniques, allowing her to effortlessly access military files. After her father died, she sought to gain control over the Umbrella Corporation, which according to her had actually been founded by the Beardsley family alongside the public founder Oswell E. Spencer. She eventually pursued development of the Neo-T Virus.
She then created improved creatures via the Neo T-Virus, including Millenia, who was modeled after her DNA. Eventually, her plans hit a snag when the British Army counterterrorism CRW member Norse Chirac as well as Rain Rubens arrived at the castle and uncovered evidence towards her experiments. After meeting them at a dinner party, she then had Rain abducted, and then lured Norse (who was searching for Kate) into a trap using Rain as bait. She then personally arrived at Rain's holding area, and after having her restrained, then offered her "soup" that included human eyeballs, which resulted in Rain screaming and Norse following the source of the scream while holding off two snipers. Afterwards, Mylène then deliberately unleashed a biohazard on her staff, turning them all into zombies, and most likely infecting the guests. This ended up causing Gilliam to turn against her and forward a memo to an Umbrella executive notifying Mylène's selfish behavior, as well as being given the command of "taking care of her."
Mylène then directly contacted Norse inside the lab, and after relaying her past to him, she proceeded to undo Rain's restraints and send her to the room Norse was in. She then deployed various culture tanks containing monsters, eventually deploying Millenia. She ultimately was killed by Rain shooting the Government handgun at Millenia.