Play as Shadow Kirby (Multiplayer and Training mode only)
Beat "Very Hard" in solo mode to unlock Shadow Kirby, you can die as much as you need so as long as you beat very hard you will unlock Shadow Kirby.
Alternate costumes for abilities
Play and beat any difficulty in solo to unlock a second costume for a ability.
Play as Beetle,
Kirby & play on Waddle Dee Train Tracks Bell
If you own or have borrowed a copy of Kirby: Triple Deluxe you can allow to put the games streetpass on your 3DS which will allow you to access Kirby Fighters Z's hidden content, Beetle, Bell Kirby and Waddle Dee Train Tracks.
Alternate Title Screen
Unlock all second costumes and Shadow Kirby to get a new title screen which will feature all the Kirby colors and the majority of abilities with their second costume on, King Dedede will come up pop up after a while.