Dragon's Dogma/WMG

Pawns are The Heartless, only not evil; Arisen are Nobodies, only also not evil.
The Pawns are created from hearts consumed by the dragons. They basically imitate life because that's all they remember, but they have no wills of their own. Likewise, Arisen have iron wills that allow them to continue living without their hearts, but no true emotions. Pawns only act and feel human in the presence of Arisen, because they literally complete each other.

The Brine is actually a sentient hive mind-type being.
And it is allied with the Arisen. It can't communicate with humans or pawns, though, so its attempts to save people from drowning are viewed as attacks. Also, it feeds on pawns, which is why they fear it so much.

Alternatively, the Brine is a sort of adventuring tool created by a previous, hydrophobic Arisen.
Said Arisen was tired of his enemies washing up on shore, so he created a creature to destroy anything he submerges beyond a certain depth. As a failsafe, he attuned the creature to the Arisen's scar, and also Saurians because he had one as a pet when he was younger.

The Brine is the spirit of the former royal lifeguard.
One day he fell asleep at his post, and the duke's 2nd wife drowned. He was executed for his failure, and now his spirit haunts all deep water, rescuing those that fall in, hoping to atone for the one life he failed to save.