Here's a rundown of the hidden characters and features in the game along with the requirements to unlock them (there's a whole bunch of unlockables)

The Different Types Of Sonic

* Dark Type Sonic - 100 Silver Medals
* Fire Type Sonic - 105 Silver Medals
* Wind Type Sonic - 110 Silver Medals

Unlock Party Characters
Collecting Fire Souls will unlock the four hidden characters playable in the Party Mode.

* Blaze - 105 Fire Souls
* Silver - 087 Fire Souls
* Cream - 042 Fire Souls
* Shadow - 030 Fire Souls

Unlockable Awards
Do the required to unlock the award in Chapter 13/14.

* Bookworm - Play a total time of 24 hours
* Champion - Win 1st in 120 Party Games
* Dark Master - Beat 20 missions using at least 5 Dark skills and no Wind/Fire skills
* Extreme Speeder - Use Speed Break a total of 50 times
* Flame Master - Beat 20 missions using at least 5 Fire skills and no Wind/Dark skills
* Genie Buster - Defeat a total of 1,000 Genies
* Grind Performer - Grind a Distance of 30Km (18.64 miles)
* Hero - Defeat Erazor Djinn
* Rebellion - Beat Erazor Djinn at level 25 or lower
* Ring Catcher - Collect a total of 10,000 Rings
* Skill Collector - Unlock every skill available
* Skill Quinti - Beat 20 missions using 4 or less skills
* Skill Saver - Beat 30 missions using less than 100 SP
* Super Player - Reach level 99
* The Ultimate - Collect all Gold Medals
* Time Controller - Use Time Break for a total of 300 seconds
* True Hero - Defeat Alf Layla wa Layla
* Wind Master - Beat 20 missions using at least 5 Wind skills and no Fire/Dark skills
* World Traveler - Travel a total of 500 km (310.7 mi)

Unlockable Party Mode Extras
Collecting Fire Souls will unlock another difficulty for the Computer players and other extras in Party Mode.

* Aim! Big Crossbow - 006 Fire Souls
* Cross! Balance Race - 048 Fire Souls
* Extreme Difficulty - 114 Fire Souls
* Final X Adventure - 126 Fire Souls
* Fly! Ptero-Rider - 078 Fire Souls
* Put It Out! Big Fire - 060 Fire Souls
* Reel! Battle Fishing - 024 Fire Souls
* Row! Canoe Race - 018 Fire Souls
* Sink ‘Em! Pirate Ship - 012 Fire Souls
* Spin! Propeller Race - 096 Fire Souls
* Thrust! Bone Knight - 036 Fire Souls
* Watch Out! Trap - 054 Fire Souls
* World Bazaar - 069 Fire Souls