The following characters become unlocked when you complete the corresponding task.

Commander Video:
Collect 8 golden skulls.

Sugimoto:
Collect 16 golden skulls.

Juni:
Collect 18 golden skulls.

President Thompson:
Collect 20 golden skulls.

Zombie:
Collect 23 golden skulls.

El Testigo:
Collect 26 golden skulls.

Curly Brace:
Collect 30 golden skulls.

Jim Hawkins:
View the 'True' ending.

Tina Hawkins:
Successfully complete Ukampa Ruins.