I would imagine it as individuals entrapped in a psych ward with similar dementia that feeds into the lifeblood that made 'wonderland'.
'Alice'-type characters would be ensnared by the paranormality that the realm's magic created, parasitically depending on the 'Alice'-types for strength to exist...though likely exploited by interior nefarious means. While the actual Alice may no longer be a contributing factor, there could be candidates that could be reached through dream-states in order to pull them into the madness which cements their place as new 'Alice's.

This is purely speculation, of course. Just spitballing ideas to rationalize more than one 'Alice'-type character, should people want to play as them.

- Never played the games, but heard of the plot.
- May be confusing a few aspects with Wizard of Oz.

EDIT: However, the more contribution there is to a creation, the more the fears and strengths come out. Feeding off of what they need, the forces of antagonism grow stronger with every 'Alice'-type pulled into wonderland.