What are messengers and how to I awake/unlock them?

[ZIGS]

In some puzzles, there's a wall where you can paint a call of help but the reply says I need to awake a messenger before he can help me. What are they and how do I find them?

[Fabian]

You need to get to building C there there is an axe, get the axe get back to building A or B break the wooden door, solve the puzzles behind the "wooden" door and you get a messenger