BECOMING JADE by Tyree Campbell

Annae (real name Maryjade) is an assassin sent to Deege, a forested world, to kill a plant and bring back the druzy who carries it. Druzies resemble young girls, but seem to have no life and no purpose but to act as transportation to the plants. In the process, Annae loses contact with her own spacecraft and is marooned on the world. 
 
The man who hired Annae for this task is also responsible for the death of Annae’s twin sister. Annae has accepted this contract because it presents an opportunity to kill the killer. However, the loss of the twin has crippled Annae. She is virtually unable to communicate with anyone, except in the course of negotiating her contracts. She has taken to talking with the memory of her dead sister, and with no one else.
 
Now, marooned on Deege, she must find a way to break out of her isolation and communicate with the druzies, and with a strange young woman who cannot speak, or she will be compelled to remain on this world forever.