When a Human loses is original form, (That is, reincarnates in the soul in its pure form), it must necessarily link itself with any monster, so as not to disappear completely. Connection helps the soul stay alive.

To establish a connection, the monster must touch an object that contains a part of the human soul, such as A Chara`s pendant, or Violetta's apron (the Soul of Kindness).However, you need to do this (touch the object) shortly after the Human's death, neither earlier nor later.

A monster can become an "ally" of a Human either voluntarily or forcibly. Although, in fact, most people have connected themselves with the souls of Humans only out of ignorance.

Toriel and Violetta got along very well, which is why, when Violetta's strength was running out, she asked her to take her apron. This is just one example, the rest will be shown and explained later...intrigue!

No one can see the soul except the one with whom it is connected. The monster and the human must be close to each other.