Soul Link System is a device developed by Dr. Gero that allows both natural and artificial souls to be linked to another person allowing said soul to control them. The system is a major plot device in the Super Warrior Arc, Enemy Warrior Arc, and Android 21 Arc of Dragon Ball FighterZ.
At some point during his life, Dr. Gero developed a technology that allowed him link souls to another person giving the soul control over that person's body. This technology was apparently left for use by Android 21, the human mother of Dr. Gero's deceased son Gebo converted into a Bio-Android incorporating the cells of both fighters and scientific researchers.
When Android 21's evil personality went out of control, she utilized the Soul Linking System to link artificial souls to Android 18, as well as the revived Frieza, Cell, Nappa, and the Ginyu Force.
In order to stop the evil Android 21, Android 16 uses the system to link a human soul with Goku during the Super Warrior Arc. However during the Enemy Warrior Arc, the Soul links with Frieza instead though this allows the soul to keep Frieza under control and leads to the revived villains forming a truce with the Dragon Team to defeat her.
However in the Android 21 Arc, Android 21's original personality regains control after her evil personality activates the Power Suppressing Wave Machine. As a result, she and Android 16 decide to link the soul to Android 18 as they plan to have the soul grow strong enough to be linked with Android 21 to help control her insatiable appetite and supress her evil side.
One of the advantages of Soul Linking is the linked soul is not effected by the Power Suppressing Waves thus Soul Linking allows the soul or the fighter their linked to to utilize their abilities. The stronger that link the more power they have access to as shown by Goku and Vegeta who after achieving a strong enough link gain access to their Super Saiyan Blue forms. However it is possible for a fighter to bend an artificial soul to their will as done by Frieza and Cell during the Super Warrior Arc and by Cell in the Enemy Warrior and Android 21 Arcs. However human souls are stronger and harder to control, though even they can be overcome as shown by Android 21 when her hunger went out of control after using her True Form while Soul Linked to defeat Cell.
Gods and divine beings such as Beerus and Whis are able to detect linked souls as both quickly noticed the soul linked with Goku during the Super Warrior Arc. The Soul can communicate with the person they are linked to in a mental space though the fighter cannot harm the linked soul and the linked soul can refuse to act leaving the fighter's body immobilized which the soul can use to keep fighters like Frieza in check. However both the soul and the linked fighter work best if they can come to an understanding.
It is implied that Dr. Gero may have intended to use the Soul Linking System as a potential method to controlling Androids like 17 and 18 as Android 21 demonstrates that 18 could be controlled using an artificial soul. Alternatively, it was intended to be used with his Cloning technology as the clones are mindless thus linking them to souls would allow them to be controlled by the linked soul, however, Android 21 never does due to using them as her food source. However, she does link artificial souls to the revived villains which suggests the same could be done to their clones. Like Android 21's purpose, it is never revealed what Dr. Gero or his Supercomputer's original plans for the soul linking system were.
Video Game Appearances
The Soul Linking System was introduced in FighterZ where it plays a key role in the game's plot as the main protagonist is a unnamed Human Soul linked to Goku during the Super Warrior Arc, Frieza during the Enemy Warrior Arc, and Android 18 during the Android 21 Arc. In addition to Goku, Frieza, and Android 18, the soul can link with their allies (or accidentally with Cell during the Super Warrior Arc) as well.
In Dokkan Battle, Soul Linking appears during Story Event "DRAGON BALL FighterZ: Super Warrior Arc". Unlike FighterZ, the Soul is given the same name the player uses for the unseen Time Patroller protagonist from Quest Mode. The Story Event follows the plot of the Super Warrior Arc.
In Super Dragon Ball Heroes: World Mission, it is called the Link System and plays a role in the Arcade Mode Universe Mission Age Android 21 Saga: Activation which adapts the Super Warrior Arc and Enemy Warrior Arc. If the player has their Hero Avatar on their team, their soul will be linked to either Goku or Frieza allowing them to control them for the rest of the battle. Additionally Soul Linking grants some Ability Effects in addition to giving the Avatar access to the body and Super Attack of either Goku or Frieza.