Simple! You must complete the game and get the Darkrai extra mission by using Wifi! Then when you do it and after you get to somewhere else (after team dim sun trapped you and darkrai you got out) He will be your partner!
Manaphy and Darkrai.
in Pokemon ranger 2 i would say darkrai/heatran/garchomp/or charizard.
i think you can only get it at a Pokemon event or with Pokemon ranger 2 (with out cheats.)
You need to complete Pokemon Ranger 2 and the "Liberate the Tower" extra mission, and 2 DS's (one with Pokemon Ranger 2,one with D/P). Trade the Darkrai at the "Send a Pokemon" option,go into MYstery Gift on D/P and collect Darkrai from the man in the Poke Mart.
You have 2 Nintendo DS then send the darkrai (You have to have the mystery gift in diamond or pearl)
No
He is in the Altru Tower (possibly only AFTER Darkrai)
Darkrai
You can't have a Darkrai as your friend pokemon, I think, but the final boss at Altru Tower is a Darkrai.
Aerodactyl
Darkrai
A Quest on Boyleland after you've defeated Darkrai in Pokemon Ranger: Shadows of Almia.