Veshta

'Yuuko' was not this Sea Hag's real name, since at one point, she bargained for this one with the real Yuuko. Ever since Kokoro traded her own name in exchange for her lover's, this malevolent crone wasn't known by any other moniker - other than her transient titles and stolen names - until it was revealed that she called herself Veshta. Whether even that was true is up to debate, but it is the best that we've got. 

Alongside her sisters, 'Kokoro' and Gimansha, she terrorised the Unseen Village on the Verge, touching and corrupting many lives there. The latter two were slain during the quest What Does the Fox Say?, but Veshta managed to escape, and started brewing up her next schemes in hiding. As of Fox Tales, her current whereabouts were unknown, but during Hag Hunt: Heart's Revenge, she was discovered in Aburayan Springs, on the southern coast of the Millennium Forest, where she was confronted and finally defeated for good. 

All three Unknown sisters preferred the illusory disguise of beautiful koi-fish-looking Mermaids, since their real forms were hideous.

The Unknown of the Unseen Village called them 'Ladies of the Sea', and didn't know about their true nature, implying that there might be - or might have been - real Merfolk around their island. 

During Hag Hunt: Heart's Revenge, Veshta, now disguised as Mistress Bernadette of the Aburayan Bathhouse, appeared as a fair and ephemeral Cecaelia, though her machinations were unraveled, and she was confronted in the subterranean tunnels and caves beneath the Bathhouse. 

Both on the Verge and at Aburayan Springs, Veshta commanded Fuaths and Boggards. When she had to escape to the Millennium Forest, some were loyal enough to follow her, and while the majority were eventually slain one way or the other - or simply abandoned her - two stood out as her lieutenants. 

Veshta's theme song 1 and theme song 2. 

Felmire dismembered the Hag's corpse upon her defeat, claiming her clawed hands and her eyes as grisly trophies for the Lodge in First Landing.

Other items of note that Veshta carried upon her person were Mesmerising Opals; a spotless Mudlily; a number of Runes carved into a piece of driftwood, a smooth pebble, and an exquisite example of skrimshaw; and finally, a Relic of Fire and Water in the shape of an ear-like golden seashell, which she used for communication with Lord Azar Hacofyre when making her wishes. Her mastery of the magic of the sea depths granted the Water aspect to the Relic.