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Mario 2 before it became Mario 2!
Played as Mario's equivalent, Imajin.
Watch Highlight: Slacker Sunday: Doki Doki Panic (Imajin playthrough) from spoonshiro on www.twitch.tv
The other three characters are Lina (Peach's equivalent), Mama (Luigi's equivalent) and Papa (Toad's).
Page on the Mario Wiki explaining the story and the changes made between versions.
And a bit on Wikipedia explaining how YK:DDP became Mario 2.
And finally, the TCRF Page showing side-by-side version differences.