Before Kingdom Hearts Union χ
Like the rest of the Dandelions, Elrena is sent to the new plane - "Unchained" - before the Keyblade War, surviving the conflict and forgetting about the world she was leaving.
Then, Elrena began to relive her past experiences in given versions of worlds with the rest of the Dandelions, while having the Keyblade War erased from her memories. Thanks to this, a double of Elrena - constituted from the recorded data of what she saw again in the virtual versions of the worlds - was created.
One day, while she is sitting alone in one of the streets of the Daybreak Town, Elrena is approached by Lauriam, who asks her if she is Elrena. After she confirms her identity, Lauriam asks her what she knows about Strelitzia. Elrena confirms that she knew Strelitzia, but only in having gone on missions with her. She claims Strelitzia was quiet and seemed to always be waiting for someone, so she didn't disturb her. Elrena summoned her Chirithy, who added that he and Strelitzia's Chirithy saw her go into a building with Skuld.
Once Lauriam departed, Elrena entered the building in question to investigate. Her Chirithy noted this was unusual behavior for Elrena, as she never cared much about other people.
After Kingdom Hearts Union χ
At some point, Elrena's heart fell to the darkness, becoming a Heartless. Elrena's Nobody, Larxene, came into being, where she was found by Xigbar and brought before Xemnas, who was looking for ancient Keyblade wielders to fill slots in Organization XIII. After Larxene's defeat at the hands of Sora, Elrena is recompleted, but will become a Nobody again to join the Real Organization XIII because of the "Ancient Keyblade Legacy that would slumber within her".