Relationship between Keyleth and Vax'ildan

During his near-death experience when he first encountered the Briarwoods, as his life flashed before his eyes Vax'ildan thought of Keyleth as being "beautiful, walking under the trees". Vax later revealed to the druid, after almost dying again at the hands of Professor Anders, that he was in love with her and emphasized it with a kiss. This greatly flustered Keyleth, and she later admitted to having some feelings for him as well, but she revealed that she was afraid of pursuing the relationship. As such, Vax agreed to "wait for her", to give her the time she needed to make her choice as to whether or not she wanted him.

As their adventures continued and after the trauma of the Chroma Conclave's attacks across the world, Keyleth became increasingly receptive toward Vax's companionship, allowing him into her room the night before the party left Greyskull Keep, as well as kissing and hugging him after Grog's test of character against Earthbreaker Groon in Vasselheim.

After Vex'ahlia was killed and resurrected during their search for the Vestiges of the Divergence, Keyleth later explained that she was scared of losing Vax and, while she thought she loved him, she felt as if she was too terrified to allow herself to because of her fear of outliving and losing everyone once she completes her Aramente.

Over time—and after having been given encouragement from her father Korren, Vex, and Pike—Keyleth realized that those fears were ultimately unfounded, purely based on an unwritten future. It all came to a head with the aftermath of finding the body of Tiberius Stormwind in the ruins of Draconia, the dragonborn having been killed by Vorugal. After saying her farewells to Tiberius and burying him, upon returning to Whitestone, Keyleth eventually went to Vax's room and insisted on being with him for the night.

During this time, as Vax poured his heart out to her and expressed his doubts about the two of them and where they stand, Keyleth interrupted him, telling him about her conversation with Pike and the realization that they ultimately had "nothing to lose." Having said that, she finally confessed that she loved him and had been for a long time, apologizing for taking so long to say it. Vax then admitted again that he loved her as well. They then spent the night together.

After witnessing Keyleth nearly die in battle against Anna Ripley and her henchmen on the Isle of Glintshore, and after Raishan made her presence known to them in Whitestone, Vax voiced his fear of losing her and his thoughts about her extended longevity should she complete her Aramente. Nevertheless, he wanted to ensure that it would happen.

After killing Vorugal, Vax voiced his objections to Keyleth's desire to go after the Diseased Deceiver right away without the others, convincing her that they needed the rest of the party to do it and that despite her misgivings about the dragon's inevitable betrayal the longer they waited, he assured her that they wouldn't have to wait long.

Vex'ahlia's Reactions to Keyleth and Vax'ildan's Relationship
Vax didn't talk to his sister about his love for Keyleth, which caused some tension between them. Vex also, for the most part, seemed to resent their potential relationship initially and made a point of disturbing their private moments. When they returned to Vasselheim, she teasingly reminded them that Kashaw had kissed Keyleth. When Kashaw returned to Keyleth's life, however, Vex wasn't as resentful as expected and instead tried to encourage the druid to make her move with Vax, which she did when she openly kissed him after Grog's test of character against Earthbreaker Groon.

Vex continued adjusting to the reality of her brother's feelings, and after the party survived the slaughter of Umbrasyl, she asked Vax what was going on with him and Keyleth, but Vax refused to answer. Vex then asked how Keyleth feels about him, and Vax replied, "Ask her." So, Vex promptly asked Keyleth, who reluctantly admitted to having feelings for Vax and asked if Vex would hate her for that. Vex, after her death and resurrection, however, was no longer resentful and encouraged the druid to just be happy and not live in fear. Since then, she still teases her brother about their relationship, but she also hasn't shown any further resentment, admitting to him that it makes her happy to see him happy.