Before diving into engagement techniques, we need to understand the reference. Yuna, the beloved summoner from Final Fantasy X (and X-2), represents a particular brand of mature sweetness. Unlike the bubbly or tsundere archetypes, Yuna’s sweetness is:

Yuina smiled, and it was like the sun breaking through the clouds. She pulled back just enough to grab the cold cup of tea, set it aside, and then she leaned her entire weight against your side, resting her head on your shoulder. She pulled the kotatsu quilt higher, tucking it around you both.

"Ending with an older girl, more of Yuina's sweet [something?]."