Zhaan is a Delvian, which has a combination of spiritual and sexual implications. Some of the time she functions as a priest, or Pa'u, which gives her some almost magical powers which are critical to resolving the plots of several episodes. Her ability to coordinate activity and heal also consistently supports the action.
Perhaps her greatest contribution to the show, however, is her presence as a mysterious and alluring blue woman. Zhaan is remarkably unreserved about her physical features and erotic nature. As for myself, I find all the serious contact lens action that goes with her role to be the most captivating part.
Passing through a system with intense solar radiation, Zhaan finds herself distracted by photogasms. Her reaction to the solar flares makes D'Argo and Rygel nervous, and limits her capacity to contribute for the duration as well. There is the implication at the end of this episode that Crichton would return again to experience the solar flares, and it doesn't seem unreasonable that Zhaan might be motivated to accompany him.
In an episode that focuses on Delvians and Zhaan's unique abilities she shares minds with Crichton, and is able to lean on him to provide the essence she requires to rebuild the foundations of her, at that time, shattered personality. A recurring theme with Farscape is to make use of mechanisms like this mind sharing stuff that are familiar at best and tired at worst, but to present them along with enough plot twists and quality performances to enliven the material. This business of an arguably more advanced being using a mind share with a lesser being to provide an essential mental service is a new twist on this old idea, at least as I can reacall, and the way it is handled in this Farscape episode was more compelling than I can remember any such scene being before. It had me coming back to rethink what happened after the episode was over.
Kind of tangential, but in one episode Crichton becomes unstuck in timeand ends up spending a lot of time crushing a decorative mask of Zhaan's while talking in her quarters. At first he accidentally breaks it, but then he purposfully steps on it, which makes for an intense scene. Zhaan, of course, takes all this with composure.
Zhaan is played by Virginia Hey, who has a website. Lots of folks have gone there and said they like it, but I have avoided it myself because I don't like having to register to get free content. From what I've heard she's not even blue in most of the photos on the site, but I'm not sure what to make of that.