Mermen have one of the most gentle dispositions of any monster. Most of them are hopeless romantics, and they're always waiting for their true love. They appear at beaches and on top of reefs, and dreaming of their yet unseen “princess” (or prince depending on their sexuality). Their singing voices contain magical power, and humans who hear their songs are completely charmed and drawn to them.
Also, their blood has the effect of dramatically increasing the lifespan. It is said that it has this function to make their partner's lifespan match her own, and without exception, their human will be made to drink their blood. Because of the effect of the “merman’s blood,” it is in extremely high demand among humans seeking eternal youth and longevity, and other monsters who need to extend the lives of their humans in the same way.
Mermen are actually able to move on land as well, either by crawling on their tails, learning spells of human transformation, or learning how to use their mana to hover off the ground (though this doesn’t mean they can fly freely through the sky).