Support characters can be summoned with a TIGBox, a throwing item. Each appears for a few seconds, executes some attacks, and then disappears.
Here is a list of all planned support characters.
Cabadath (Trilby's Notes, 6 Days a Sacrifice)- Teleports behind another character and delivers a powerful slash with his scythe.
Sexy Hiker (Sexy Hiking)- Sexy Hiker slowly lifts his hammer, then brings it crashing down in front of him, dealing massive damage.
[image in game]Akuji the Demon- Fires a single powerful projectile.
Kittey (A Game with a Kitty)- Kittey floats briefly, and then charges forward a short distance. Then he turns around, and charges the other way. He repeats this sequence of attacks a few times.
Mia (Eternal Daughter)- Charges her sword, then releases a powerful attack with good range.
Karoshi- wanders around. If he is attacked, he explodes, leaving items behind.
Kaltos (The Spirit Engine 2)- Charges his sword, then stabs the ground, which erupts with lightning that travels to the edge of the platform and remains for a second.
Asha (Iji)- Teleports around like crazy, occasionally attacking with a flying downward stab or firing a huge plasma beam.
Rilo Doppelori (Noitu Love 2)- Fires some missiles into the air. Then they come back down in random locations (marked by crosshairs).
Gunlimb- shoots straight downwards, launching himself into the air.
Juni- Harmlessly wanders around. Leaves an umbrella behind.
Noitu Love- Immediately unleashes a flurry of punches.
Beethro (Deadly Rooms of Death)- Slowly swings his sword to one side, then the other.
John Defoe (Chzo mythos)- walks around, stabbing any who get too close.
Damsel (Spelunky)- Heals any nearby character.
Allergic Pig (Gesundheit)- Walks about, spewing a deadly snot-ball every so often.
Alien Hominid- Shoots lasers repeatedly in front of him.
Meatboy- Runs around and makes everything he touches slippery.
Shmup Storm- A swarm of dozens of ships flies across the screen, Unown-style.
Shotgun Ninja- Jumps around, tossing grenades in all directions, and firing his shotgun.
Rope Burner (You Have to Burn the Rope)- Leaps off the top of the screen and Burns The Rope, causing a chandelier to fall across the level.
Ogmo (Jumper)- Suggestions are welcome.
Marisa (Touhou)- Unleashes a devastating danmaku bullet pattern.
Tim (Braid)- Freezes time, stunning all other characters.
Runman- Dashes across the screen several times, damaging those he contacts.
Assassin Blue
King Mubbly (Frozzd) - Flies from one enemy to the next and shoots 5 balls of fire at each opponent, swaying while he shoots. The shots are somewhat random so its not completely accurate.
Sumotori - Pushes characters around.
Viridian (VVVVVV)
Iwanaga
Glum Buster - Launches characters by drawing a triangle in front of him.
Aggroll (Voltorometer) - Moves forward, damaging all he touches.
Ian (Soldexus)
Frogatto - Shoots his tongue in front of him. If he hits a character, he grabs and swallows it, doing minor damage. After a second or so, he spits the character out in front of him.
Creeper (Minecraft) - Blows up when it gets close to someone.
Dominic and Ina (Action Fist) - Fire back-to-back in both directions.
Davis or Julian (Little Fighter 2)
Commander Video (Bit Trip Series)
Dwarf (Dwarf Fortress)
Tela (Guardian of Paradise)
The Dishwasher
Flip Hero (Hero, Hero 3D, Hero Core) - Flies around, shooting horizontally at enemies when he is lined up with them.
Josef (Machinarium)
Rose (Rose and Camellia)
Raptor (Dino Run)
Spewer
(All of Our Friends are Dead)
Sarah (The White Chamber)
Other Possibilities:
Nautilus (Aquaria), Jill (Mighty Jill Off), The Kid (I Wanna be the Guy), Ninja/Bot (N/N+), Bird (An Untitled Story), Joe (Streemerz), Phillip (Space Funeral), Recette (Recettear), Houkaimura, Octodad, Tekkyuman, Super Crate Box Guy