- “My memory has all but faded completely... But I will always remember how much she (Lumine) too, loved these flowers.”
- — Dainsleif in Teyvat Chapter Storyline Preview
Choosing to play as the male protagonist, Aether, makes Dainsleif and Lumine former traveling partners. In the World Quest, Bough Keeper: Dainsleif, he cryptically talks of a traveling partner who he refers to as "her". Towards the end of the Archon Quest: We Will Be Reunited, Lumine appears before Aether and Dainsleif, blocking their attack on an Abyss Herald, revealing herself as the Leader of the Abyss Order. Here, it is officially revealed that Dainsleif and Lumine do indeed know each other. Lumine however, considers Dainsleif to be her enemy due to the fact that she is the current leader of the Abyss Order, Dainsleif's main opponent.
Given from Dainsleif's expressions however, he appears to feel betrayed and hurt by Lumine declaring him as her enemy. Lumine tells Aether that Dainsleif was a royal guard of the final dynasty of Khaenri'ah who failed to prevent its destruction 500 years ago. She also tells Aether that he should not team up with Dainsleif. Aether then tells Lumine to return home with him, but she refuses. Lumine then departs into a portal shortly after that and Dainsleif dashes after her, following her into the portal.
Apart from the game's quests, we also get a possible glimpse into their relationship in the Teyvat Chapter Storyline Preview. In this video, Dainsleif acts as the narrator, going over each of the 7 nations. Towards the end of the video, he directly addresses Aether: "Step forth, if you have understood the meaning of your journey. Defeat me, command me to step aside, show me that you are worthier than I to rescue her." At the very end he quietly muses to himself how most of his memory has faded but what he can never forget is "how much she too, loved these flowers." It is also worth mentioning that the flowers in the Sea of Flowers at the End are identical to the ones Lumine wears in her hair.
Paimon: "That does sound pretty dangerous... Maybe they're [Abyss Order] looking for a better time to ambush us or something..."
Aether: "They won't defeat me/I'll defeat them."
Paimon: "Uh-huh! We're super-duper strong!"
Dainsleif: (That manner... it's just like hers.)
-Bough Keeper: Dainsleif
Paimon: "Uh, you know, Dain, you keep talking about that person this, and that person that..."
Paimon: "Where are they now, anyway?"
Dainsleif: "She... *sigh*
Dainsleif: "No longer travels."
Dainsleif: "I'm afraid that traveling can be... too exhausting at times."
- Bough Keeper: Dainsleif
Dainsleif: "Lumine. We meet again."
Paimon: "Wh—What's going on here? Dain knows her, too!"
Lumine: "You shouldn't team up with this man."
Lumine: "He is my enemy."
- Chapter I, Act IV: We Will Be Reunited - "A Soul Set Apart:"
Aether: " What is the history between you and my sister?
Dainsleif: " We were travel partners."
Dainsleif: "We both partook in a painful journey of searching for our fate, but regrettably... we did not make it to the journey's end together."
- Chapter II, Act IV: Requiem Of The Echoing Depths - "In the Depths, an Unexpected Reunion:"
This ship had started after the Bough Keeper: Dainsleif quest. This quest had given the players the idea that Dainsleif was a traveling companion of the Traveler's Sibling. And with the content of the Teyvat Chapter Storyline Preview, that had only strengthened the Romantic Implications. A twitter artist named june had created a comic called "A memory from long ago" which follows this ship back to when the two were traveling together. The comic was later dubbed RedVelvetVA and Sylleblossom.