You get a top-secret super Pokemon for completing the dex.
This, of course, adds another Pokemon to the dex, so by obtaining that Pokemon, you'll be completing the real dex. The prize for that is another secret Pokemon, which completes the real real dex, earning you another secret Pokemon which completes the real real real dex, which gets you a diploma.
I claim to be the official Porygon2 of the Pokemon Black 2 board. Scientist MileRun: WaRy associate