Nathyrra is only the sleeping man's true love if she is in your party when you are speaking to the knower of names.
The dialogue between them is pretty funny - well you know how drow females can be - if you want to be spoiled SPOILER COMING UP -
she gives him a slap and tells him to sling his hook, then says that he's fun and she might keep him
The other alternatives for the Sleeping Man's true love are Aribeth if she is in your party, and I think if you have neither of them then the knower of names says it is the Sensai (who you killed earlier).