@mase_aga
From what I remember (read some Greek myths back then) sirens are monster girls with avian features or fish features. The avian variant is overshadowed by harpies in works that has mythological creatures, while the fish variant overtime became synonymous with mermaids. Harpies were used by Zeus to punish Thracian king Phineus by eating his food and/ or shitting on them, starving him until the Argonauts arrived. Both variants of harpies sing hypnotic songs to lure sailors to crash their ships against rocks and drown them. Not sure about the differences between the fish variant sirens and mermaids though.
Another fun trivia: The name
siren inspires the scientific name for the mammal order Sirenia, herbivorous marine mammals like dugongs and manatees. This is because it is believed sailors saw the silhouettes of these animals and thought they were sirens/ mermaids.