The person we play as in the Beginning Hour demo purposefully has no identity because he's not a canonical character. The demo is just intended to provide a flavour of the final product. The character, what happens to him and the LSD ending is just for the purposes of the demo. The only canonical aspects are the VHS tape and the location, both of which will feature in the final game.
As for the story, there's obviously more to Mia than meets the eye. She allegedly went missing in 2014, the same time the Baker's did. That's her on the ship at the beginning of Tape 3. Whatever that ship is, I believe it will loosely tie into previous events in the series. We also know from the helicopter photograph in the demo that Umbrella Corps will play a role in the game. Although confusing, it's important to note that 'Umbrella Corps' has nothing whatsoever to do with Umbrella Corporation. Umbrella - Spencer's Umbrella, do not feature in this game at all. Instead the 'Umbrella Corps' are a special forces unit belonging to an as yet unidentified organisation. Biohazard: Heavenly Island (The Japanese manga sequel to Marhawa Desire) suggests this organisation is Sheng-Ya Pharmaceuticals; a Chinese corporation briefly mentioned in the Umbrella Corps game. The plot of Heavenly Island also takes place in 2014 and fills in the backstory first hinted at in the files from Lost in Nightmares. It takes place on the same island Spencer sent Alex Wesker to to create the immortality virus. Instead she worked on the 'Kodoku Project' which was a survival of the fittest approach to creating the perfect B.O.W. - interestingly using the exact same method the Hypnos Gene was created in Gun Survivor to create the Hypnos T-Type Tyrant. Two operatives from Sheng-Ya are sent to steal this new research and it seems no coincidence the story of Heavenly Island takes place the same year the Baker family and Mia eventually disappeared.
It's also interesting to me that the Baker's have extraordinary regenerative abilities whilst still remaining realistically human. Aside from the obvious insanity, they appear to be the perfect representation of what Oswell Spencer sought all along - immortality and meta human. That was the whole purpose of Umbrella Corporation and the t-Virus project after all.
