Rhonda is a kind person who has a pretty good sense of humor; usually using unexpected lines. It's reasonable to believe that she is a good cook as well, having made grilled cheese sandwiches and tomato soup for the children in the first book. There's no proof that she's the one who cooked, but it's reasonable because Number Two and Mr. Benedict are constantly busy and Milligan is the only guard/gardener/handyman in the house at the beginning. She is assumed to be somewhat motherly and is extremely worried when Number Two and Mr. Benedict (her adopted family) go missing.
Rhonda has an excellent intuition (she knew she was in a maze immediately, for example) and can memorize letters after reading them a few times. This is reminiscent of Sticky's ability, but it is likely that she processes things at a slower rate (which would still be well above average). She has also proven to be an excellent driver in the third book, when she and Number Two defeated a Ten Man by knocking into him with a fake ambulance then chaining him up. She is also fairly adept at keeping Sticky, Kate, Constance, and Reynie in line.
All that we know of Rhonda's history is that she was born in Zambia, moved to Stonetown when she was a child, and can barely remember her native language (Bembi). In Mr. Benedict's Book of Perplexing Puzzles, Elusive Enigmas, and Curious Conundrums, Rhonda is seen with Mr. Benedict winning an award at a spelling competition next to Reynie and Miss Perumal. During the series she makes a ridiculous appearance as a green-haired girl with a big, pouffy dress. She, along with Number Two and Constance, was adopted by Mr. Benedict during the series.