What's more to say? She Ranma's mom, the one which Genma deserted and then claimed that had died. Well, she's back, albeit rather late in the series. She's quite easy to spot too, since she's always carrying a sword, and is very dangerus in that respect - she can't handle the sword very well. She did not know that Ranma actually had a split personality early on because Genma did not want her to know that her son was no longer a true man. If she discovered the truth, she would have had to force both Ranma and Genma to commit ritual suicide, then slit her own throat to join her kin. The reason for this was a promise Genma made to Nodoka when he departed, which was to make Ranma a real man. This was a task he felt to be impossible if Ranma stayed in his mother's presence and convinced Nodoka to let them proceed on their journey. Until they admit their true identity to Nodoka, Genma will have to remain as a panda and Ranma as Ranko, Akane's "cousin". One of the running jokes whenever Nodoka is around is that waiters will always trip and spill hot soup on father and son; taps and wells supplying cold water would also either be out of order or all dried up respectively.
SPOILER: Nodoka eventually found out the truth, but because Ranma had acted bravely in recovering the Saotome family heirloom - which his father had stolen to sell away - she was convinced of his manlihood and both father and son were spared. The duo were then brought back home, but it was later wrecked by Shampoo and they all had to move back to the Tendo household.