Dimension Links

Though you spend the majority of the game playing as only Terra, Ventus, or Aqua, that does not mean you are alone. As you visit the different worlds, you will make strong bonds of friendship with other characters. These turn into Dimension Links (D-Link for short). This will open their abilities for you to use almost any time you want to. Each character you D-Link with has their own set of abilities for you to use, as well as a Command Style. As you use a D-Link, you will gain a star shaped flower . This will then level up your D-Linked character, giving them new abilities and commands. The first one is pretty quick to show up, but the second flower might not come for quite some time.
You can keep a linked person for as long as you have D-Link Energy. This is displayed in the gauge above the HP bar. When this runs out, you will lose your connection. Almost every enemy drops little blue prizes that refill your gauge. Also if you leave the area, you will lose your link. So make sure, especially if you are trying to level them up, to elliminate every enemy in that specific area.
Also you can access all of your D-Links via the main menu. In here, you can see what level that each D-Link is, what abilities they have, and what Commands you will have. You can also turn them on and off through the menu. This is a big help, since the commands change as you level up, and you might not catch your D-Link being leveled up while fighting.
Using D-Links is not really necessary to getting through the game, but you will miss out on some really great abilities and finishing moves that only your friends have. I would strongly suggest getting each one leveled out, and using them.
Below is the complete list of possible D-Links each character can gain. Just look above the tables to see who will get them, since Terra, Ventus, and Aqua will not always get the same D-Links.

Ventus, Aqua

	Finish		A strong jumping slash blow.
		Overdrive	Attack power increases when HP is at less than 25%.
	Landbreaker		Breaks the ground so that nearly all enemies are damaged and blown off.
		Auto Block	Automatic guard against enemies from the front.
Com. Decks		Lv.0: Quick Blitz, Sliding Dash, Stun Edge Lv.1: Quick Blitz, Sliding Dash, Stun Edge, Sliding Dash, Zero Gravity Lv.2: Quick Blitz, Sonic Blade, Stun Edge, Sonic Blade, Zero Gravira, Binding Strike, Cure

Terra, Aqua

	Finish		A speedy rapid combo attack.
		Haste	Increase your speed and the speed of normal attack.
	Air Dive		Press , , , or to hit enemies while flying through the air.
		Auto Counter	When blown away by enemies you will automatically counter attack.
Com. Decks		Lv.0: Quick Blitz, Strike Raid, Aero Lv.1: Quick Blitz, Strike Raid, Sliding Dash, Aero Lv.2: Blitz, Strike Raid, Sliding Dash, Blitz, Freeze Raid, Sliding Dash, Aerora, Cura

Terra, Ventus

	Finish		Shoot many magic orbs towards enemies.
		Magic Deflector	Sometimes guard against magic automatically.
	Magic Volley		Shoot magic orbs. Sometimes there will be a chance to press , then you can teleport and shoot more magic orbs.
		Auto Life	Auto Revive from battle once with 25% health.
Com. Decks		Lv.0: Blizzara, Thundara, Cura Lv.1: Blizzara, Fira, Thundara, Magnera, Fire Strike, Cura Lv.2: Blizzara, Fira, Thundara, Mine Square, Blizzara, Magnera, Fire Strike, Cura

Terra

	Thunder Chaser		Hold and use directional key to move thunder cloud to an enemies head and strike them.
		Focus Saver	Decrease Focus Gauge consuming rate.
	Dragon Fire		Hold to shoot a massive fire breath in front of the user towards ground area.
		Drain	Drain HP when performing any attack against enemies.
Com. Decks		Lv.0: Fire, Binding Strike, Blackout, Sleep Lv.1: Fire, Fira, Ignite, Binding Strike, Blackout, Sleep Lv.2: Fire, Fira, Firaga, Ignite, Binding Strike, Dark Firaga, Blackout, Sleep

Terra, Ventus, Aqua

	Dream Sparkle		Raises enemies up into the sky by a magic light. Press when the indicator reaches the coach to raise attack power.
		Auto Recovery	When receiving a bad status, it will automatically be recovered after a few seconds.
	Miracle Dance		Spin, making a magic vortex. Press when the indicator reaches the coach to raise attack power.
		Regen	Restores HP gradually from time to time.
Com. Decks		Lv.0: Wish Circle, Fairy Step, Fairy Heel Lv.1: Wish Shot, Wish Circle, Fairy Step, Wish Circle, Fairy Step, Fairy Heel Lv.2: Wish Shot, Wish Circle, Fairy Step, Wish Shot, Wish Circle, Fairy Step, Fairy Heel, Fairy Heel

Ventus, Aqua

	Sweet Memory		Create whirlwinds of flowers that attack enemies.
		Gauge Boost	Increases the Command Gauge speed.
	Sweet Seven		Create seven colored jewels surrounding you and sending them to attack nearby enemies.
		Quick Load	Deck commands reload faster.
Com. Decks		Lv.0: Sleepy, Grumpy, Doc Lv.1: Sleepy, Sneezy, Grumpy, Happy, Doc Lv.2: Sleepy, Sneezy, Grumpy, Happy, Dopey, Bashful, Doc

Ventus, Aqua

	Lightcharge		Spinning attack that continues as long as the player successfully matches button prompts, dealing 150% damage each hit.
		Auto Teleport	Teleport behind enemies when targeted (50%)
	Faithcharge		An enchanted version of Lightcharge that fires orbs as Mickey spins, dealing 150% damage each hit.
		Double EXP	Earn double EXP and CP from defeated enemies
Com. Decks		Lv.0: Sliding Dash, Sliding Dash, Binding Strike, Cura Lv.1: Sliding Dash, Sliding Dash, Binding Strike, Binding Strike, Mine Shield, Cura Lv.2: Sliding Dash, Binding Strike, Sign of Faith, Binding Strike, Mini, Mine Shield, Cura

Terra, Ventus, Aqua

	Fruit Shot		Summon a giant fruit to bash into your enemies.
		Munny Plus	Earn 2x the munny from defeated enemies.
	Rumble Rave		Whirl around in Captain Dark's kart and crash into enemies.
		Steal	Occasionally steal items from enemies.
Com. Decks		Lv.0: Taunt, Confusion Strike, Binding Strike Lv.1: Taunt, Binding Strike, Confusion Strike, Confetti, Aerial Slam Lv.2: Taunt, Binding Strike, Confusion Strike, Confetti, Binding Strike, Confusion Strike, Aerial Slam

Terra, Ventus, Aqua

	Heroes Entrance		Jump upwards and slash downwards creating a beam wave.
		Berserk	Increases Critical Hit Rate, but greatly decreases Defense.
	Heroes Pride		Shoot a beam wave and perform many teleporting slashes.
		Attack Deflector	Randomly guard automatically when attacked.
Com. Decks		Lv.0: Fire Strike, Quick Blitz, Fire Strike, Quick Blitz, Slot Edge Lv.1: Fire Strike, Quick Blitz, Fire Strike, Quick Blitz, Stop, Slot Edge Lv.2: Fire Strike, Blitz, Fire Strike, Quick Blitz, Fire Strike, Blackout, Stopra, Slot Edge

Terra, Ventus, Aqua

	Random Beam		Fire a beam that bounces off walls and enemies, dealing 200% damage. Add more shots by hitting buttons when promted.
		Attack Boost	Increases power of Attack Commands.
	Ohana Beat		Creates musical waves that deal 330% damage, and stun enemies. Can add up to 8 more hits by hitting the buttons when promted.
		Double CP	Obtain more CP from defeating enemies.
Com. Decks		Lv.0: Thundara, Thundara, Zero Gravira, Stun Edge, Cura Lv.1: Thundara, Thundaga, Thunder Surge, Zero Gravira, Stun Edge, Cura Lv.2: Thundara, Thundaga, Thunder Surge, Thunder Surge, Zero Graviga, Thundaga Shot, Stun Edge, Cura

Terra, Ventus, Aqua

	Swordbill		A long distance lung that to a seven part combo if it connects. The lunge deals 150% damage while each subsequent hit deals 200% damage.
		Float	Decrease the speed at which you fall to the ground.
	Barrel Roll		A mid-air barrel roll that can be repeated up to four times by tapping random buttons when promted, dealing 220% damage per hit.
		Double Strike	Basic attacks deal double damage.
Com. Decks		Lv.0: Quick Blitz, Quick Blitz, Sliding Dash, Confusion Strike Lv.1: Quick Blitz, Quick Blitz, Sliding Dash, Confusion Strike, Zero Gravity Lv.2: Blitz, Quick Blitz, Quick Blitz, Sliding Dash, Confusion Strike, Zero Gravira, Cura

Ventus, Aqua

	Donald Flare		Launches an assortment of fireworks, both duds and live, that deal 230% damage. Can continue the fireworks by hitting the buttons when prompted.
		Magic Boost	Magical damage is increased by 50%.
	Cosmic Donald		An enchanted version of Donald Flare that increases the damage of the blue ones to 300%, and adds red ones that deal 600% damage. May also cause Ignite (50%)
		Shell	Fire, Blizzard, Thunder and Dark type magic is halved.
Com. Decks		Lv.0: Firaga, Blizzaga, Thundaga, Fire Dash Lv.1: Firaga, Blizzaga, Thundaga, Fire Dash, Fire Surge, Cura Lv.2: Firaga, Fission Firaga, Blizzaga, Thundaga, Fire Dash, Fire Surge, Ignite, Cura

Ventus, Aqua

	Goofy Spin		An accelerated spin attack that the player can control with the analog stick. Lasts five seconds and can deal 200% damage 45 times.
		Protect	Halves all physical damage.
	Goofy Turbo		An accelerated ricocheting rocket attack that the player can control with the analog stick. Deal 320% damage up to 56 times.
		Stun Boost	Stuns foes when you hit them with a basic attack (75%)
Com. Decks		Lv.0: Sliding Dash, Strike Raid, Stun Edge, Confusion Strike Lv.1: Sliding Dash, Strike Raid, Stun Edge, Confusion Strike, Freeze Raid, Confusion Strike Lv.2: Sliding Dash, Strike Raid, Stun Edge, Confusion Strike, Freeze Raid, Confusion Strike, Strike Raid, Sonic Blade

Ventus

	Last Word	Surround Vanitas with crystallized beams of radiant light.
	Dark Link	Changes your Shoot Lock to "Dark Link"
	Com. Deck	Lv.0: Dark Spiral, Dark Splicer, Dark Spiral, Dark Splicer

Note:Vanitas's D-Link is plot driven in Ventus's final battle of his story. It cannot be obtained in any other way.