Leomon

First enter Gear Savanna, head east and then north, and you will see a Patamon in trouble!  You will see a Leomon take care of the bad guy for you.  Then he headed for the Gear Savanna training area.  Talk to him, and he will tell you that he is looking for something, and that he can't go with you to the city until he finds it.  You tell him that if you find it.  Leomon is looking for his ancestors' sacred tablet, which can be found in the Drill Tunnel.   When you give him the tablet, he will join the city.