Taissa and Van
As the most constant romantic relationship in both timelines, Taissa and Van's romance meets a devastating and untimely end in season three. However, Van refuses to leave the world before reuniting with Taissa after realizing that Other Taissa had been in charge for a while. It is romantic, the idea that Van's greatest success in a treasure hunt would be bringing the love of her life back to the surface. Still, it is tragic that they could not be together until Van's cancer eventually won out, especially seeing as Van's end comes right after an active choice not to kill Melissa.
In the adult timeline, Taissa and Van's storyline is all about reconciliation, and a love story with the tragic understanding that they are running low on time. There will not be a long-term romance in the cards for them either way, so they may as well enjoy the time they do have left. Van and Taissa indulge in childhood whimsy with a dine and dash from a fancy restaurant and then question just how much of the Wilderness sacrificial rituals could be reality when a waiter's death is immediately followed by finding out that Van's cancer is improving. They have years of missed opportunities to make up for and memories to create, such as a nice payoff to Van's admission that she would one day like to take Taissa to New York City to get a big pretzel and take a horse carriage ride.
In the 1997 timeline, Van and Taissa's relationship is present but not as front and center in the plot. They get a few significant moments, such as Van trying to bring out Other Taissa to have her pull the trigger against Ben, and when Van and Taissa admit that Van has been practicing to rig the cards. Still, they do hold important parts in the overall plot as they witness the power struggle occurring around them. As individuals, Van and Taissa continue to be essential to the ensemble group of characters, and their relationship helps to ground them. Taissa and Van's plan to rig the hunt to give Hannah the queen card unfortunately goes haywire when Shauna realizes that there is a plan and interrupts it, which finds Van later on being devastated that their actions had accidentally led to Mari's death rather than the intended target.
Still, while the 1997 iterations of Taissa and Van set the foundation of their relationship, it is their older counterparts who truly take the lead in the dynamic between the characters in season three. Van's death also triggers a change in Taissa as she reverts to cannibalistic tendencies, ingesting Van's heart as a way to have them remain connected forever, while plotting against Shauna with Misty as a way to avenge Van's death. While Other Taissa is a part of the character, Yellowjackets has yet to see Taissa truly lose control, and this may be the final straw that breaks Taissa's calm exterior and transforms her into a more feral character, letting her join the ranks of Shauna's growing murderous tendencies and Misty's history of kidnapping.