Mareel was a Tyran female who was native of the planet Khefka IV. Mareel grew up on the streets of Khefka IV and later worked in an accommodation house there, where she met Verad; the two fell in love and she then accompanied him to Trill.
In 2370, she visited Deep Space 9 to help abduct the Dax symbiont from Jadzia Dax so that it could be transferred to Verad. Despite the warning by Benjamin Sisko that Verad was sure to be greatly changed after joining with the Dax symbiont, becoming a mixture of Verad, Dax, and all of Dax's previous hosts, Mareel believed that he would continue to love her, and she him.
Unable to convince Verad to return the Dax symbiont to Jadzia, Sisko attempted to convince Mareel, urging her to talk with Verad and see for herself how much he had changed. She refused, but eventually began to see the truth in Sisko's words, after she noticed the distance that Verad began putting her at. Mareel, nevertheless, continued to plan her escape with Verad, but when Verad changed their plans and told her that she should wait for him at a designated rendezvous point instead of him waiting for her, she realized that he had, in fact, changed despite his assurances.
After Verad and T'Kar prepared to leave, Sisko again tried to sway Mareel, who confessed that Verad lied to her for the first time and that she feared he would not meet her as planned. Taking advantage of her sadness, Sisko reminded her that the old Verad still cared about her, and that by saving Jadzia, Verad would be saved as well.
Mareel ultimately agreed to help Sisko rescue the symbiont. After it had been removed from Verad, he felt troubled and alone, but Mareel assured him she'd always be with him. (DS9: "Invasive Procedures")