From time to time, you can battle Kukui at the end of the Pokémon League. But if you’d prefer to meet him outside of battle, you can simply visit his lab on Route 1. The first time you speak to him, he’ll reward you with a Lucky Egg if you’ve registered at least 50 Pokémon in your Pokédex.
This valuable held item boosts the experience points from defeating a Pokémon by 50%, making it easier to Level Up your Pokémon. Also, if you leave the lab and return, the Professor has another gift for you: TM90 Substitute. As explained, it creates a decoy by expending 25% of the user’s maximum HP.
If you visit the lab after completing Episode RR, Kukui and Lillie will divulge the findings of their research on Z-Moves to you. Afterwards, if you talk to Lillie, she will offer to become a Multi Battle partner in the Battle Tree. This time around, she will assist you in battle with a Comfey and Ribombee.
That’s everything you can do on the ground floor. If you head downstairs and approach the punching bag in the nearby corner, you’ll receive a surprise visit from Kukui’s old friend, Molayne, and his Machamp. You don’t get anything from this visit, but it’s nice to see Molayne again, outside of the Pokémon League anyway.