Beginning of the mystery: https://www.minecraftskins.com/skin/19394341/my-secret-minecraft-mentor/

First clue: https://www.minecraftskins.com/skin/19451760/my-secret-minecraft-mentor-clue-1/

Second clue: https://www.minecraftskins.com/skin/19603521/my-secret-minecraft-mentor-clue-2/

Clue #3: The number of hashtags in the previous clues are the amount of characters in his username