You're parallel versions of the same character, defined by (number of people playing) traits and related NPCs. For each PC, one of those traits is different. Why? Parallel NPCs can be as different as you like. Whose character can travel between realities? How? The PCs start in a discreet spot, with another Doppelganger. They've just met: who called them? They all have one Urgent Need another Doppelganger can resolve and a Secret they want to hide from the others. Throw a pile of pick-up sticks on the table. To succeed at an uncertain action, a player must remove one (the GM decides which): - Easy action: a stick with 2+ stripes - Medium: with 3+ stripes - Difficult: with 5 stripes or a spiral Each time the pile moves, it disturbs the multiverse: 1: Another PC benefits from the success. 2: The active PC swaps locations with a Doppelganger not present. 3: A trait of the active PC is modified (the player chooses which, the others choose how). 4: A hostile Doppelganger appears in the story. 5+: All Doppelgangers randomly swap locations. You can put removed sticks back on the pile to counter perturbations (one per rank). No available sticks? The multiverse implodes.
An expanded version of my entry (in French, sorry!) can be found here: https://i.imgur.com/WeEewjb.jpg