Dimension Link

The Dimension Link (ディメンションリンク), also known as D-Link, is a feature of summoning in Kingdom Hearts Birth by Sleep. Its name refers to its role as a link to summon a character's power from another world.

Kingdom Hearts Birth by Sleep
Aqua makes three Wayfinders, and she gives two of them to Terra and Ventus the night before she and Terra take the Mark of Mastery exam. After telling the legend behind the Wayfinder, she explains that she added some magic to the ones she made, representing an "unbreakable connection".

As the three of them journey from world to world, the bonds they make with those they meet along the way create D-Links, through the power of their Wayfinders.

Usage
A Dimension Link can only activated when the blue D-Link Gauge is full. The gauge is located on the left of the HP gauge, and it is filled by picking up blue D-Link prizes, or by using a Balloon Letter. To activate a D-Link, press on the D-pad and select an enabled link from the list.

Activating a D-Link immediately restores all lost HP. While the link is active, the equipped Command Deck is replaced with the deck associated with the link. Additionally, the Command Gauge changes its appearance to reflect the linked character, and a sprite of the character appears above that of the player character. Upon filling the Command Gauge, the player can use a unique Finish command. While the link is active, the D-Link Gauge decreases continuously, and the link is broken when the gauge is completely empty. Alternatively, the player can deactivate the D-Link early by selecting "Revert" from the D-Link menu, by leaving the area, or by bringing up the main menu.

During a D-Link, any defeated enemy will have a chance of dropping an emblem, and collecting it will raise the D-Link's level, expanding the Command Deck and activating an ability, represented by the symbols to the left of the linked companion's sprite. Bringing the link up to Level 2 expands the deck again, adds a second ability, and grants access to a more powerful Finish command. Once a D-Link has reached Level 2, any additional emblems collected simply restore the D-Link Gauge.

The player is able to create D-Links with their friends from the Mirage Arena over a wireless connection. Linked friends appear as Terra, Aqua, or Ventus in their respective armor, and the associated Command Decks and Finish command will be the ones that were equipped when the link was made.

D-Links can be disabled from being used in-battle from the main menu, under "D-Links". This can be used to shorten the D-Link menu, making it easier to select the desired link quickly.