In Ni no Kuni Soulmates are parallel versions of a character in different worlds that share the same soul (or "soulbond"). If one dies in their world, the parallel-world version of themselves dies as well. The same is true for becoming heartbroken (a term used that essentially means "emotionally unwell" due to excess or lack of specific emotions, often caused by black magic).
Differences between soulmates
As seen in the game, most soulmates/parallel versions of characters share age, gender, and general demeanor. Some characters have changed "species", but in the second world the difference between species is more like a difference between nationalities in the first world. Job titles or status can differ between the worlds, for example Leila owning a milk bar in the first world, but being royalty in the second. There are no fairies in Oliver's world, or in The Conductor's dream world (although, in the NDS' version of the dreamworld there are no other characters at all besides the Conductor himself).
Interestingly, soulmates can subconsciously recognize people they haven't met if their counterpart has met them. For instance, after Oliver met Myrtle, Esther declares that she felt like she met him before.
|This section contains plot details, which may spoil your gameplay.|
List of known soulmates
|Ni No Kuni||Motorville|
|King Tom||Timmy Toldrum|