Hermione Granger was found out as a mermaid in her first year when she was knocked out of the boat into the black lake 4 people had to help her get out of the water they saw her tail and helped try to dry herself.
Hermione was sorted into Ravenclaw she didn't make any real friends they just wanted to know how she become a mermaid she never told anyone not even her parents she did help Harry Potter to study his magical education because he would need it in the future.
She would usually would swim in the black lake when she isn't studying, Homework or in a class. on Halloween night after a charms class with Professor Flitwick, a frightened Quirrell ran in and informed everyone that was there that an escaped mountain troll has entered the castle. As the houses are led back to their common rooms, Harry and Ron tell Professor McGonagall tell that Hermione is missing.
Hermione was going to go for a late night swim. Her head of house will be the only one she would tell she was a mermaid, she told him all the facts of being a mermaid and because of the Full moon she would get a single room. She told him of some of her powers when she was walking she saw the troll she entered the bathroom that she passed by.
The troll smelled a human, Hermione and went after her he tried attacking her smashed bathroom stalls and the sinks were smashed. Hermione got an idea once she saw the sink on the ground she opened her hand and the water followed it and she splashed water on the troll and made a stiff hand motion and the troll froze over as an ice sculpture.
The staff were running to the bathroom when the heard roaring and crashing they ran to the door of the bathroom and saw a ice sculpture of a mountain troll and saw Hermione on the ground Professor McGonagall exclaimed "explain yourself" she just replied back by saying that she saw the troll on her way back to her dorm room.
The next she heard was that Quirrell died he tried to Kill Harry and he somehow died and the Philosopher stone was destroyed.
