Kasumi is one of the few kinder souls of Ranma 1/2, as her personality radiates politeness and friendliness. She almost never gets mad, but if necessary, she'll scold someone to get a point across. (I do remember when she made violent attacks, but that WAS when the oni possessed her) Her duties in the kitchen takes up almost all of her time, so she is oblivious to the events going around the dojo.
As well, she is a bit of a ditz. She's not clumsy, but oblivious to the most obvious things. She spends most of her time in the kitchen, so she is unaware of the events around her. This, of course, provides the dramatic relief, which is necessary for some of the episodes.
Of course, she and Dr. Tofu have feelings for each other, but both are too shy to speak up. When Dr. Tofu sees Kasumi, his glasses fog up and he gets very clumsy, and Kasumi is amused by that. This was the focus of several episodes in the original TV series.