REAPERS: eat the humans. they're soiling the good name of merpeople.

VENGEFULS: fighting the Reapers, hate them, trying to regain the honor of their species.

GUARDIANS: keeping the humans from running around screaming about how they're slowly being eradicated by merpeople.

HUMANS: caught in the middle. protected by Vengefuls and Guardians, preyed on by Reapers.

Guardians can leave their forest pools and mingle in the river if they want, but they must return to the forest at some point to continue watching over travelers.
All merpeople can go on land a maximum of forty eight hours, then they must return to the river for at least ten hours.
Being on land wears down a merperson very fast. It takes all their energy to retain their shape and also retain all the liquid their body needs to survive.
The merpeople know all about how the humans live, don't try pulling some Little Mermaid stunt.
It is almost impossible to tell a merperson from a human while the merperson is in human form. The only factors that might give them away are hair color and fin placement.
Merpeople can only change their tails into legs, and since most of them have fins on their backs, they must wear clothing that covers them at all times or they will be seen.
Humans don't care if the merperson is a Vengeful or a Reaper. They will try to kill it.

Any more questions, feel free to post on the questions board or PM an administrator!
This is a very lax type of roleplay, and merpeople are extremely diverse, so if its not listed somewhere, it might still be allowed.