It is easiest if you make it warm up to you using a soothe bell or any other way. Then you raise it's level. You can get the Exp. Share which is a quick and easy way, or you can do it the hard way, battles. Then, Congrats, you have Lucario!
Where to find
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Soothe Bell- In the Pokemon Mansion (below Hearthome, on route 212)-
go to the second room on the left side and talk to the maid. She will give it to you.
Exp. Share- Go to Eterna City and go to cycling road. In Between Cycling Road and Eterna there is a man in white clothing. He is Lucas' Father. You need at least 35 Pokemon in your sinnoh dex to get it. Talk to him and you will get it.
(all of this is in the south part of Eterna City).
-For Pokemon Diamond/Pearl
Rioulu evolves with happiness during the day.
you have to beat drake form the elite 4
I think the fastest way to evolve rioulu is to put an exp share on him then go to Pokemon league and fight the Pokemon he will get tons of exp and he evolves into lucario at level 19 so it shouldn't take that long
All I know is that you need to use an item to evolve Rioulu
Riolu evolves not from a level, but from happiness. It must evolve in the daytime. So when its happiness is maxed out and its daytime, when it grows up a level, it should evolve.
Riolu is evolved when levels up with high happiness during daytime.
He has to be happy by 12:00 Pm. At Daytime. If you need help search it up @ http://www.bulbagarden.net/
Simply follow the story line because when you reach the end of iron island you get an egg. when you hatch it, it is a rioulu Good Luck!!!
you go to jublife city get on a boat this girl will ask you to go in a moutain with her(make sure you save)then at the end shell give you a egg.it will hatch to rioulu
if you put it in 1st place
If your riolu is following you in heartgold or soulsilver you can talk to it.
make riolu consume solar ribbon when the voice asks if ya need an item to evolve