To unlock Han, you need to find all 5/5 characters from Star Wars and 100,000 coins. I played as Bazooka Trooper because he has Enemy Access and bazooka for breaking glass or metal things.
C3PO - Lost Temple
Go to the room with golden skull, blow up metal barricade, jump into another room and break the barrel.
Luke Skywalker - Into the Mountains
After completing some tasks, find breaking ice paths and break glass barrier.
Chewbacca - City of Danger
Before the end of the level, find this door, solve the puzzle and use kid to go through the hatch.
Leia - Free the Slaves
Find this place, shown on the screenshot, blast metal barricade and open the door.
R2-D2 - Desert Ambush
In the beginning of the level, use horse as Indiana to jump on this cliff and open the hatch by whip.
Bonus! Santa Claus - Into the Mountains
After leaving the Marion's Tavern go to the small house on the left and trick the guard to get inside. Santa will fall from chimney and leave a trail of studs.