A bit of theorycraft. I just tried Nanasu with her touch of sleep.
She instantly puts anything that tries to attack her in melee to sleep for you to execute it with one guaranteed crit, constantly gaining eye of mind exp, which is otherwise a pain in the ass to train.
Even if it somehow survives, it gets a talisman blast to the face and probably a second sleep.
So, touch of sleep is great. And Farris' lullaby too.
And here we have Undine, who has both. With a rare race, so in theory she can be turned into a healer or anything else you need.
The only problem is her base speed is 60, but if you're going to wear her as a hat all the time, that doesn't matter. Which is what you logically should do, since she's a mermaid and shouldn't be walking on land normally.
Lulwy's faith bonus and speed rings are static anyway, so 40 speed difference aren't that important if you're not going to ride her.
She also has an innate 20 nerve resist, so you can convert her to Kizuami for holy+nether resists and speed.
The only slots she's missing are waist and legs. You could even go full degenerate and give her a fox tail and hooves, If you want.
In theory, it makes a good build. The mermaid you're carrying constantly sings, lulling everyone to sleep. If enemies get too close, she stops them with a touch of sleep. All you have to do is kill them with crits.
As for the class, you could choose something with a mind domain to maximize sleep and confusion, or just go full sword sage for damage. And the mermaid could learn mind arrow and some powerful healing spell.