The Soultouch is a phenomenon that only occurs in the Echidna race.
Whether it is genetic or mystical is unknown, but its effects are well-recognized. The Soultouch is designed to allow Echidnas to know when they've found their mate in life, the identification of which is made simply by laying eyes on them. When only one of the pair is aware of it, it can cause an attraction that causes that one to follow the other until such time as the other also realizes the Soultouch. It is known that the Soultouch does not guarantee a lifelong relationship, and in fact some couples bound by the Soultouch have split up. However, even broken up couples remain connected by the Soultouch, to the point where one can get a vague sense that the other has died through it. (KtE: #26)