Unlock Tails or Sonic and Tails
Have the Christian Whitehead 2013 update installed and then playing as Sonic complete the game collecting ALL the Chaos Emeralds.
Start a 'No Save' file. Tap the letters in order when the SEGA logo appears so that a chime plays. Then when the 'title' screen appears hold two fingers on the bottom of the screen and slide them up to the top of the screen. If you have done this correctly the 'Level Select' option will appear.
Unlock Debug Mode
At the 'Level' select screen play songs 01, 09, 09, 01, 00, 06, 02, and 03 in that order from the Sound Test. Then when you start a level tap the GUI in the top-left corner of the screen to activate/deactivate Debug mode.
Have the Christian Whitehead 2013 update installed and then complete the game with ALL the Chaos Emeralds to unlock Knuckles as a playable character.