It's my first time playing tavern and spear and I was doing the lizards vs bulls quest, then I remembered there was the black demon monster that I couldn't defeat before and when I defeated it, after some dialogues I was introduced to Meko a ghost(?) In a form of a horn. Am I supposed to know who this character is cause the dialogue seems to suggest that.

In short answer, no

We just look like his lover

To my knowledge, Meko is a character that may be known at a later time when more of the story is published, but is currently only known as a bull who used to be Eyvind's partner in a previous life.

Note that my information may not be thorough as I haven't played this game much or kept up with its story recently.