Requirement: Defeat the Emperor in his castle
To recruit Lauren you have to find him and convince him to join. Now usually the hard part is to convince someone but in this case finding Lauren is much harder as you seem to always be a step too late in chasing him around the world. Here are the steps you have to take till you actually meet Lauren:
1) Talk to the soldier in the northwest area of Imperial Castle Town just west of the entrance to the Imperial Castle.
2) Talk to the guy just next to the Porter in Harva Town.
3) Talk to the guy next to the Inn in the east in Sandora Town.
4) Talk to the woman next to the Porter in the Shrine of Mave.
5) Talk to the guy on the pier of the harbor in the northern part of Dea Town.
6) Talk to the pirate in the house next to the weapon shop in Port Town of Nex. It has to be night to be able to speak to him.
7) Talk to the child next to the bar in Prose Village. It has to be day or the child won’t be there.
8) From Harva Town sail north till you reach the passage that leads to the center of the island. Along this passage there is a stream to the east that you couldn’t pass before. Do it now and enter the Scholar’s Hut on land. (Also grab the treasure)
9) Talk to the guard in the northwest of the Imperial Castle Town again to get the final destination.
10) Teleport to Mount Egma and go to the summit where you finally meet Lauren. After a conversation he decides to join your team.