Steadfast be bossin' out of Lucario's Abilities!
All the abilites that Lucario has are to do with Flinching except for Justified. So, Steadfast makes Lucario a perfect Para-Flinch counter, as the Flinching raises it's Speed. Inner Focus only stops anybody from Flinching Lucario, nothing else. Steadfast is the way to go. You can actually try Justified too, if you want it to kind of be a Dark Type counter. This doesn't work very well though, just stick to Steadfast.