Dimension Link

The Dimension Link (ディメンションリンク), abbreviated to D-Link, is a feature similar to summoning that appears in Kingdom Hearts Birth by Sleep. It allows the user to summon another character's power from across the worlds.

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 be activated when the blue D-Link Gauge is full. The gauge is located directly above the HP gauge, and it is filled by picking up blue D-Link prizes, or by using a Balloon Letter. The protagonists can activate D-Links by pressing on the D-pad and selecting an enabled link from the list. D-Links cannot be activated if there are temporary allies in the party.

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.

While the player character is using a D-Link, defeated enemies have a chance of dropping a special emblem (シンボルマーク) that can be collected to raise the D-Link's level, expanding its Command Deck and unlocking a unique ability, as 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. 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.

In the PSP releases of the game, the player can create custom D-Links based on their friends' characters at the Mirage Arena, using the PSP's wireless connection. This feature is not present in the Kingdom Hearts HD 2.5 ReMIX release due to the removal of multiplayer. These D-Links can also be removed or replaced at player discretion. The custom D-Link will operate as a variant of the Terra, Ventus, or Aqua D-Links, depending on which character their friend is using, but its Command Deck and Finish Command will match whatever commands the friend had equipped when the D-Link was created. Regardless of level, these D-Links' sprites are an armored version of the character the friend is playing as. Like the standard Terra, Ventus, and Aqua D-Links, it can be leveled up by collecting emblems, unlocking the appropriate abilities as well as any Advanced or Ultimate commands that were part of the Command Deck. However, the player cannot use any commands that their character wouldn't be able to use in normal gameplay (e.g., Terra cannot use a Faith command within the custom deck).

Mirage Arena D-Links
In the original PSP releases of Birth by Sleep, players could D-Link with other players via local play within the Mirage Arena to obtain their Command Deck and Finish Commands as unique D-Links that can be used through the game. Due to a lack of a multiplayer function, this feature was removed in the Kingdom Hearts HD 2.5 ReMIX' version.

Notes and references
Lien dimensionnel Dimensions-Link