The protagonist is or becomes intricately bonded to someone or something else in a manner much deeper than friendship, loyalty, love or such.
Alternate names: Soul pact, Psychic bond, Empathic bond
The first video game about Soul bond was released on August 18, 1999.
Commonly the bond can't be broken, and if it can, the effect tends to be devastating to both.
A lesser variant of this is the mage's familiar, but even they can be as profound as what this tag is for (though usually they are not).