Play as The Intruder - Successfully complete Story mode to unlock The Intruder.

Accomplish the indicated achievement to get the corresponding number of Gamerscore points:

Frenzyfest Master (15 points): Come first in a Frenzyfest Tournament.
Well Fed Fish (25 points): Accumulate 5000 fish in your food bank.
Fast Food (20 points): Complete Time Attack!
Life of the Party (15 points): Compete in Party Games to accumulate 1000 Party Points.
Mega Frenzy (5 points): Rapidly eat fish to max out the FRENZY meter in a single player game.
Well Defended (10 points): Accumulate three simultaneous fish shields.
Barracuda Bully (25 points): Score a 'Barracuda Bonus' by biting a barracuda's tail four times in a row.
School Teacher (20 points): Eat 500 entire schools of fish, or swarms of bugs.
Flippin' Good Time (15 points): Complete a Quadruple Flip.

Additionally, there are three secret achievements, all of which are currently unknown:

Secret achievement (10 points): Unknown.
Secret achievement (15 points): Unknown.
Secret achievement (25 points): Unknown.