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

Brody:
Collect 350,000 coins.

Frank:
Collect 40,000 coins.

Fresh:
Collect three Boomboxes.

Frizzy:
Collect 150,000 coins.

King:
Collect 80,000 coins.

Lucy:
Collect 7,000 coins.

Ninja:
Collect 20,000 coins.

Prince K:
Collect 980,000 coins.

Spike:
Collect three Guitars.

Tagbot:
Collect 12,000 coins.

Tasha:
Collect 30,000 coins.

Tony:
Collect 95,000 coins.

Tricky:
Collect 3 Caps.

Yutani:
Collect 3 Spaceships.

Zoe:
Collect 120,000 coins.