When you complete certain missions hidden characters are unlocked in the game. Below is a list of the characters that can be unlocked and the mission which needs to be completed.

Unlock Gum.
In Jet Grind Radio's world complete mission #12.

Unlock Meemee:
In Super Monkey Ball's world complete mission #8.

Unlock Amy Rose:
In Sonic the Hedgehog's world complete mission #2.

Unlock Shadow The Hedgehog:
In Sonic the Hedgehog's world complete mission #14.

Unlock Pudding:
In Space Channel 5's world complete Mission #1.

Unlock Gilius:
In Golden Axe's world complete mission #1.

Unlock Reala:
In NiGHTs' world complete mission #2.

This hidden court which comes with a 'Tagging' game is unlocked when you beat Level 1 of Jet Set Radio game.

If you want more playable characters in the game beat the indicated tournament to unlock them.

Unlock Amy:
Beat the Bronze Tournament in Singles.

Unlock MeeMee:
Beat the Silver Tournament in Singles.

Unlock Shadow the Hedgehog:
Beat the Gold Tournament in Singles.

Unlock Reala:
Beat the Bronze Tournament in Doubles.

Unlock Gum:
Beat the Silver Tournament in Doubles.

Unlock Pudding:
Beat the Gold Tournament in Doubles.

Complete the tasks to get the GamerScore.

Play Ball - GamerScore 10:
Win your first match.

Surf the Net - GamerScore 10:
Win a Match on XBox LIVE.

Space Channel 5 - GamerScore 10:
Watch a match on.

Starting Grid - GamerScore 10:
Win with ALL 8 starting characters.

Step into SEGA World - GamerScore 10:
Complete your first Challenge (Rank A)

After Burner - GamerScore 20:
Hit a serve over 100mph (160kph).

Master Tagger! - GamerScore 20:
Without missing a shot clear a Tag.

Mouse Mania! - GamerScore 20:
Rescue 500 ChuChus over time!

Cameo Appearance - GamerScore 20:
Hit a SEGA court cameo.

Let's Get Friendly - GamerScore 20:
Win 10 Xbox LIVE Player matches.

Classic Complete - GamerScore 20:
In one SEGA game (Rank A) complete ALL the Challenges.

Climb the Ladder - GamerScore 20:
In Tournament mode win the final stage.

OutRun - GamerScore 20:
Run a total of 10 kilometers (6 miles).

Pot Black! - GamerScore 20:
In one game clear 4 Trick Shots.

Puyo Puyo! - GamerScore 20:
Clear a 20 Puyo combo.

Dragon Slayer! (20:
In under 20 seconds destroy the Final Squilla.

Ball Buddies - GamerScore 30:
Win 5 XBox LIVE Doubles matches.

Beat them All - GamerScore 30:
Beat ALL the characters in Singles matches.

Challenges 101 - GamerScore 30:
In ALL SEGA games (Rank A) complete ALL Challenges.

Full Roster - GamerScore 30.
Unlock ALL the Secret Characters.

Magical Sound Shower - GamerScore 30:
Unlock ALL the Secret Music Tracks.

Original Teams - GamerScore 30:
Beat ALL the classic Doubles pairs.

Party Time - GamerScore 30:
Play each type of Ball Game Multiplayertime.

Play the Field - GamerScore 30:
Unlock ALL the Secret Courts.

Score Attacker - GamerScore 30:
Unlock ALL the Ball Games.

Secret Stage - GamerScore 30:
In Tournament mode beat the hidden stage.

Seeing Stars - GamerScore 20:
Enter Superstar mode 10 times in a match.

SEGA Rally - GamerScore 20:
Hit a 40 shot rally.

Super Sonic - GamerScore 20:
In one game collect 7 Chaos Emeralds.

To The Max - GamerScore 20:
Hit 3 Max serves in a row

Top Gun - GamerScore 20:
Hit 100 Aces.

Top of the Tree - GamerScore 20:
On XBox LIVE win a Tournament.

Top Ranker - GamerScore 20:
Win 10 XBox LIVE ranked matches.

Zombie Hunter! - GamerScore 20:
Hit an 8 zombie combo.

Supercop! - GamerScore 20:
Without being hit destroy Kong.

Swinging Report Show - GamerScore 30:
Become a TV Superstar.

To be this good takes Ages - GamerScore 60:
In ALL SEGA games (Rank AAA) complete ALL the Challenges.