I'm presuming you mean Darkrai, which can only be obtained through trading through GTS (although you can only seek Pokemon you have seen, so that's not the best option), but Darkrai is only available through special Nintendo Events where it is distributed, one such even has just passed in the UK.
There is a way to do it without cheating. Just simply trade a Pokemon that knows fly. Example: you have a staravia in Pokemon pearl that knows fly. Just trade it to Pokemon diamond.
Use an Action Replay or any other cheating device OR you need an item from a special event which I don't think they hold events for pearl,diamond,etc.
Yes if you want it on pearl
In Pokemon Diamond/Pearl, you find a old man at route 212 with a Leafeon.
Of course you can. I played my Pokemon Diamond with my cousin that has Pearl.
Only by cheating or hacking.
to get Manaphy without cheating, you have to have Pokemon Ranger, beat a certain mission, and then somehow transfer the Manaphy egg to Diamond or Pearl.
1.Cheating 2.(Unreleased) Nintendo Event
you can't
The action replay is a device used for cheating.
Without trading or cheating, you can't. It's a Pearl exclusive.
u can't
No it isn't cheating because that's how you complete your Pokedex in Pearl/Diamond/Platnium.
No not every pokemon. You can't catch the starter Pokemon and many of the rare pokemon, you have to trade with Diamond. sorry ...
trade diamond from Pokemon diamond onto Pokemon pearl
You trade Magmar from Pokemon pearl,diamond,or platinum to ether Pokemon pearl,diamond,or platinum.
You can transfer it from an old Pokemon game (such as Pokemon fire red) or trade other people.