Terrible Secret: Mer-vow

Inspired by the vocaloid song 'Alluring secret: black vow'.
Mermando comes in Gravity Falls lake as a heartbroken merman, whenever he meets a girl named Mabel, his life changes forever as he is desiring the forbidden love between a human and a merman. Whenever Mabel finds out that she has to be married to Gideon Gleeful, Mermando reveals his feelings but disappears when Mabel is leaving Gravity Falls for the fall. He returns 6 years later but not as you would expect...
Cover by Sharkie91