Welp, someone (the author) definitely knows their yuri tropes. :-)
Yuri indeed, although so far the "obviously" only applies to an entirely unexpected side couple. :D
It's the beta couple trope: a secondary lesbian couple introduced early on to put the hearts of readers at ease about the story having actual Yuri
, rather than just Subtext
or worse, while the main couple(s) are being setup and develop into their relationship. Later on, they can become mentor figures to the main couple.
Though this story appears to kick off with a triangle instead of a clear main couple.