Press UP(2), DOWN(2), LEFT, RIGHT, LEFT, RIGHT, B, A(2) at the "Press Start" screen. All skins will become available at the "Unlockables" menu.

Damage the following Bosses using the corresponding techniques. Defeat the boss, then the next level will then be in Hard mode.

BOSS - TECHNIQUE
Forest Shredder (Wolf Kingdom) - Reflect his dynamite back at his face.
Wolf King (Deep Underground) - Wait for him to stand on top of a breakable floor block. Hit him until laughs, then drill attack the floor so that he falls.
Axel Gear (Return To Zephyrus) - Have him get hit by the tracking purple ball attack that he uses.

To unlock Gold Sparkster mode, complete Arcade mode under the Hard difficulty setting, using the secret damage move on every Boss.

Unlock the following skins by performing the corresponding tasks:

SKIN - HOW TO UNLOCK
Axel Gear - Complete Arcade mode under Hard difficulty while using the secret damage move on every Boss.
Gold Sparkster - Successfully complete Gold Sparkster mode.

ACHIEVEMENT - HOW TO UNLOCK
Elhorn Tourist (30 points) - Completed all stages.
Zephyrus Saviour (35 points) - Completed Arcade Mode on Normal.
Devotindos Defiler (25 points) - Completed Arcade Mode on Hard in a Single Playthrough.
Legendary Possum (20 points) - Completed Gold Sparkster Mode.
Impatient Hero (20 points) - Completed all stages below par time.
Spark Dog Millionaire (15 points) - Completed Arcade Mode scoring 1,000,000 or more.
Redoubtable Runner (15 points) - Completed Arcade Mode on Hard within 60 minutes.
Ferocious Flier (10 points) - Completed 1.2 having defeated all enemies.
Wolf Connoisseur (10 points) - Completed 2.1 having collected all pickups.
Nimble Knight (10 points) - Completed 3.2 without taking damage.
Burst Friends (5 points) - Defeat 5 enemies in a single Rocket Burst or Drill attack.
Woodland Bouncer (5 points) - Defeat 3 enemies with a single reflected projectile