Ok there are two different awnsers for this 1.If you let them free at veilstone city A:They will not be at the lakes....You go straight to the top of mt.cornet 2.Help them in the diamention world A:they return to there lake and you can capture them if you want to.
You need to head to Mt. Cornet and go to the top. There you will find Spear Pillar and will have to battle Mars and Saturn while teaming up with your rival. Then you battle Cyrus and then finally You fight Dialga.
There are no 'Grand Lakes' in Pokemon Diamond and Pearl, but there are the three lakes that have Uxie, Mespirit and Azelf.
srry there isn't a trust lake in Pokemon diamond nor Pokemon pearl.
You have to free all three of them from the Vally Windworks, and then use Surf to the middle of the lake, and it should be there :)
Absolutely Nothing My Friend
I'm not sure. I don't think that getting the lake Pokemon is necessary to completing the game. Correct me if I'm wrong
you defeat team galactic in the area and save the three pokemon from cyrus
You will need to catch the three lake lengendary pokemon. (Mesprit, Uxie, Azelf).
follow Cyrus to mt.corenet
lake acuity, valor, and verity in the island caves
In order to get the three lake Pokemon catch or beat dialiga, then surf to the cavern to battle them. Mesprit however is different, It will wander about until you find it. Use wobbufet if you have national dex to get it, or save up a master ball.
the can get mesprit in lake verity,near twinleaf town , you can get azelf in lake valor which is in between pastoria and veilstone, you can get uxie in lake acuity near snowpoint city.
Celebii is not catchable in Pokemon diamond. You have to transfer it in to Diamond from other versions that have Celebii in it.