Shauna and Melissa
Shauna and Melissa's relationship was never going to end well. Shauna was far too volatile to be anything resembling a caring girlfriend. Shauna viewed Melissa as a puzzle piece to power, and was willing to lie to Melissa about the extent of her feelings and motivations. Their relationship initially stemmed from Melissa being drawn to Shauna's resilience and how she viewed becoming close to Shauna as a way to survive.
Shauna's relationship with Melissa also reinforced suspicions about Shauna's sexuality and strengthened the idea that Shauna's feelings for Jackie, among a tangled web of complicated and jealous feelings, were also romantic love. For the most part, Shauna's relationship with Melissa in the Wilderness was fairly uncomplicated. Melissa offers Shauna a semblance of closeness and permission to be exactly who she is without having to conform to fit someone else's idea of who she should be. Yet, things do complicate themselves fairly quickly once Shauna chooses a hunt over staying with Melissa after she was shot with an arrow. Shauna, unable to deny the piece of her that needs to hunt, leaves Melissa behind with the only suggestion that she cares is directing Mari to keep Melissa alive, or Mari dies.
In the adult timeline, Yellowjackets confirms that Shauna and Melissa supposedly did not reconcile anything romantic upon rescue, even in the short term. Melissa describes rescue as an event that separated her from the other survivors once they returned home. However, it appears that both Melissa and Shauna were miserable in their normal and simple lives, with each craving a sense of danger again that makes them feel alive. Still, it is a question of whether or not in the future they will be on the same side or act as opposition, seeing as their reunion found Shauna bite off a piece of Melissa's arm. While Melissa may enjoy the cat and mouse game with Shauna, it is only a matter of time before going down this road blows up Melissa's life, unless that is exactly what she wants. Melissa clearly was not against the idea of a little chaos on a daily basis if marrying the daughter of the woman the Yellowjackets eventually kill, and likely eat, was appealing.