Therein lies a tale that Jan is probably better qualified to tell. Suffice it to say that everyone prefers Julie Caitlin Brown's version of Na'toth. When Mary Kay didn't see fit to portray the character the way jms wanted it done .....
The role of G'Kar's aide was a difficult one from the get-go. Mary Woronoth who played Ko D'Ath in 'Born to the Purple' had issues with the prosthetics and refused to wear the contact lenses and so met an unfortunate airlock accident.
There being a sudden hole in the casting, somebody on the crew (I don't remember who) contacted Julie Caitlyn Brown to fill in on an emergency basis-literally overnight, iirc. In fact, her first episode was done in the other actresses mask. Because of this, JCB wasn't bound by the same 5-year contract the rest of the actors were.
After a point, JCB became concerned about the effects of the prosthetics and chemicals on her face as well as being concerned about possibly losing opportunities for lead roles that would actually show her face so she asked to be released.
But there were still episodes coming up where JMS needed an aide for G'Kar, though, so they auditioned and hired Mary Kay Adams. Word is that she nailed Na'Toth's character in the auditions but made different choices when actually playing the part, softening her. Efforts were made to nudge her back to the way the character was originally played but then she was just written out.
As stated, you will see JCB again. Not just once, either.