The mermaid had alot of friends. Those friends were the friends of the siren, who was with the sailor. The mermaid, upon hearing that the siren wasn't feeling well, decided to ask one of her friends where to talk to the sailor. It was revealed that the sailor knew the siren, and would allow the mermaid to talk to her.
The mermaid swam to see the siren, who was going through a wave of envy. The mermaid had told the siren that if she had anything to talk about, she was always there. The siren fell in love with the mermaid slowly, and with the mermaid's obliviousness of the item that the sailor and the siren had, had decided to be with the siren. The sailor found out, and got upset at her for it. The sailor is a forgiving soul though, and had forgiven the siren like he had forgiven her multiple times. He and the mermaid talk to each other more at this point, since the siren included the mermaid as much as she had included the sailor.
The mermaid had started to fall in love with the sailor, yet had no way of telling him. She had known by now the sailor belonged to the siren and so did the mermaid; in fact, the siren had told the sailor the mermaid's feelings. Unbeknownst to her, he had felt them too. Weeks later into the polyamorous relationship of the three, the mermaid invited the sailor to a guild, since she feared that nobody would accept him anywhere as he had the same interests she does. At the same time, she had invited the sea witch, her childhood friend.
The sailor and the sea witch were obligated to join, and the mermaid had made a grave mistake inviting the both of them to the same guild. The sea witch had unknowingly seduced the sailor since she had reminded him of himself when he was a wee little boat boy. She was oblivious, desperate for love, and he wanted to be the one to fill that hole, to save her. The mermaid had seen this, and felt as if the sailor would have been vulnerable. She had scorned the sea witch about this, and shunned her for a day after she found out about the demeanor. To make up for it, the sailor had introduced the sea witch to his good friend; the captain.
The sailor had also informed the mermaid about the captain, with no introduction to him. The mermaid accepted this, though a part of her had her suspicions. Each passing day, she would ask the sea witch about the captain. The sea witch was more vulnerable than the sailor... and the mermaid had realized that the captain WAS the sailor. The sea witch had told her about the size of the captain's being the same as the size of the sailor's, and since the mermaid hadn't seen the sailor talk to her nor the siren in a while, as well as the picture of the captain not being the captain...
The mermaid felt tricked. She felt appalled... even after she had made a grave mistake with the siren.