THIS IS NOTHING BUT THEORIES!!!

I am speculating that Ratoo's lover was the leader of the Axolotl cult (I am only saying axolotl, cause it looks like it XD). The one axolotl that we do see warns the Lamb that "she" went to the pier and disappeared.

The only creature we see at the pier is the Fox. Ratoo's lover either could've sacrificed herself for her cult or something happened between Ratau, herself, Ratoo and the Fox.

There is a possibility that the Fox could've confused Ratau for Ratoo. (Since there names not only look similar but Ratoo literally looks like an older version of Ratau). The fox wanted to take everything away from "Ratoo", so he took away his lover and his heart.

OR

If the Fox did mean to kill Ratau, then this could've been a way to get at Ratoo's heart even further, by taking his heart, his lover and his younger brother ( due to his betrayal of leaving the cult). The Fox probably wanted to make Ratoo suffer and be alone, so he went after his family.

OR

WHAT IF Ratoo's lover was also apart of the same cult that Ratau was in, but decided to leave by creating her own group? They both could've left, but went separate paths to protect themselves from the Fox.

This could also explain why Ratau wrote the letters to the Lamb. He wanted the Lamb to do better but was either running away or struggling to solve his own problems.

There isn't a lot of info to prove anything yet, so this is all merely just speculation.