How do you get Lucario in diamond?

Answer 1

Go to Iron Island (Which is only accessible by boat from Canalava City.), You will see a man in blue standing near the entrance (Riley).Talk to him and he'll say somthing about training and give you an HM04(Strength), then run off into the cave. Go in after him and find him. Once you do he'll ask you if you would help him get through the part of the cave. I don't know if there is a yes or no option but if there is choose yes. After you lead him through it he will thank you and give you a Pokemon egg. In that egg is a Riolu which evolves into Lucario.

Little things...

It takes 6,656 steps to hatch the egg. unless you have a Pokemon with the Flame Body ability, Then it only takes 3,328 steps.

Slugma, Magcargo, Magby, Magmar and Magmortar all have the Flame Body ability.

Riolu needs Happiness to evolve.

If you don't have room for the egg, Don't worry, You can come back and get it. Riley will be in the same spot he was before.

When leading Riley through the he will use Lucario in-battle so you can get Lucario entered in your PokeDex.

This happens in Pearl and Platinum too.