Leuce was a beautiful nymph that Hades fell in love with and dragged down to the Underworld where she lived out her mortal life and died. Hades mourned her passing and turned her remains into the white poplar tree. These trees could be found in Elysium as well as scattered around the living realm. Leuce is the daughter of Oceanus, and has no other role in mythology. Her story is somewhat similar to Persephone’s (she gets abducted after all) but she’s never referred to as Hades’ wife.
In terms of timing, most sources agree that Leuce predates Persephone, but some argue that the two were hanging out in the Underworld at the same time. Leuce was possibly mentioned as a companion of Persephone at one point, so she was was servant, and possibly a friend. I haven’t decided how to treat Leuce in my series. I traded her out for Apollo’s Daphne/Laurel, but she may come into play later. Time will tell.