Don't forget the optional threesome with her and Riona in ep 3.
I reckon that, if she is a secret LI in the event the MC doesn't end up with any of the main ones, it could be tied to her major choices. So far we've had 2 of the major choices that affect affinity that are tied to Quinn, take her number and smoke weed, and it's possible we could get more if we get a choice to help get her out of some trouble as this could also be a major choice that affects affinity.
So maybe the trigger would be that if you did the 3 or possibly more major choices tied to Quinn, then she'll get an ending with the MC but only if you don't get endings with any of the main LIs.