There are 26 masks you have to unlock for this trophy:
Richard - Rooster - Default Mask
Russell - Bull - Beside the dumpster at the end of Metro/Prelude
Rasmus - Owl - Achieve a high score on Metro/Prelude
Tony - Tiger - Achieve a high score on No Talk/Chapter 1
Earl - Walrus - Found on a dead body on the second floor in Overdose/Chapter 2
Aubrey - Pig - Achieve a high score on Overdose/Chapter 2
Don Juan - Horse - Achieve a high score on Decadence/ Chapter 3
Graham - Rabbit - Achieve a high score on Tension/Chapter 4
Jones - Crocodile - Found in a sewer behind the house, you must clear the building first and instead of getting into the car take the crowbar found on the first floor and you use it to open the sewer go inside to pick up the mask.
Dennis - Wolf - Achieve a high score on Full House/Chapter 5
George - Giraffe - Achieve a high score on Clean Hit/Chapter 6
Ted - Dog - Achieve a high score on Neighbors/Chapter 7
Carl - Grasshopper - Found in the centermost room on the first floor of Push It/Chapter 8
Rufus - Elephant -Achieve a high score on Push It/Chapter 8
Rami - Camel - Achieve a high score on Crackdown/Chapter 9
Jake - Cobra - Found in Hot & Heavy/Chapter 10, north-west section of the map on a dead body in the top most room
Willem - Monkey - Achieve a high score on Hot & Heavy/Chapter 10
Peter - Unicorn - Achieve a high score on Deadline/Chapter 11
--Chapter 12/Trauma has no mask --
Richter - Rat - Found on the top floor of Assault/Chapter 13 in the northernmost room
Zack - Frog - Achieve a high score on Assault/Chapter 13
Oscar - Mole - Achieve a high score on Vengeance/Chapter 14
--Chapter 15/Final Chapter has no mask--
Charlie - Octopus - Found on the kitchen counter in Safehouse/Chapter 16
Rick - Fox - Achieve a high score on Safehouse/Chapter 16
Brandon - Panther - Achieve a high score on Fun & Games/Chapter 17
Phil - Fish - Found in Resolution/Chapter 19, found underground lying in a box
Louie - Chameleon - Beat the game with the normal ending
Nigel - Bat - Beat the game with the secret ending