Most of them are hopeless romantics, and they're always waiting for the man who will become their husband. They appear at beaches and on top of reefs, and dreaming of their yet unseen “princes,” they sing in a beautiful voice. Their singing voices contain magical power, and men who hear their songs are completely charmed. They get dizzy and are drawn back to them, and then they end up having a destined meeting with them as their “princess.” They’re not a race that aggressively attacks humans like other monsters, however, once they do get a man, they're aggressive about sex and use their monstrous bodies to charm him. They'll strongly charm the man and will probably never let him go.
Also, their “mermaid’s blood” has the effect of dramatically increasing the lifespan. It is said that it has this function to make a man’s lifespan match her own because a mermaid’s life span is longer than a human’s. Without exception, their husbands will be made to drink this blood.
Furthermore, because of the effect of the “mermaid’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 husbands in the same way. The Merrow of the same mermaid family is actually the main source of traded mermaid blood, and its blood is in high circulation among the monsters. However, among the humans it is quite rare, so there are many humans who are after mermaids for their blood.
The mermaid in the illustration on the page to the left isn't wearing anything on her lower body, but normally their vagina is concealed by scales and can't be seen. When they start to have sex with a man, they use magic to get rid of the scales, and the vagina will appear in roughly the same position as in a human woman. Since the new Demon Lord's assumption of power, they've continued to change and adapt so that having sex with human men is convenient and pleasurable. For that reason, it's not possible for them to lay eggs and then spawn the way ordinary fish do.
- Mermaids can move on land in three ways: 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). (Source: Kenkou’s Q and A thread).