mate with the moki for that achievement
when she shows up, call her cute
Viewing post in The Spirits of Sabil (+18) comments
If you didn't call the Moki when first meeting them, they won't know you're attracted to them, and you won't be able to suggest having sex later in that save. You will be firmly platonic friends.
Spoilers below
If you did let the Moki know you were interested, you could proposition them for sex after they came back to announce that Ori successfully saved their village, and they would reciprocate your attraction and agree.