You found the baby costume.

Completed the game.

You found the miner costume.

Cleared a level with just the flamethrower.

Cleared all levels with B rank or better.

Unlocked all the achievements.

You found the clown costume.

Cleared all levels with S rank.

Cleared a level with just the throwing knives.

Cleared all levels with A rank or better.

Killed all the school children.