Rssuu was a Lahit individual from Flyspeck who was trained as a medical doctor. He was a member of Starfleet in the late 24th century. While Lahit had no sexes, Rssuu used the male pronoun for convenience for dealing with sexed species.
By 2385 Rssuu served as the chief medical officer on board the hospital ship USS Lionheart. He had himself set in a nutrient tray in sickbay, and extended his branches and vines throughout sickbay. A vocoder was embedded in his trunk which allowed him to communicate with other crew members and the voice he used had a British accent.
After Doctor Julian Bashir was rescued from the asteroid prison where he was being held, Rssuu performed an examination on Doctor Bashir, and later kept Lieutenant Seth Maslan from murdering the doctor. Rssuu then treated Bashir for theragen exposure, saving his eyesight in the process. (ST - The Fall novel: The Poisoned Chalice)