When you play Mercenary mode and get a 4 Star or higher rank in a map the following characters can be unlocked.

Unlock Ada Wong the Secret Agent:
Get a 4 Star rank in the Pueblo map.

Unlock Jack Krauser the Army Veteran:
Get a 4 Star rank in the Castle map.

Unlock Albert Wesker the Leader of Alpha Team:
Get a 4 Star rank in the Waterworld map.

Unlock Hunk the Operative for the Umbrella Corporation:
Get a 4 Star rank in the Island map.