Stella is a character that first appears during the quest: Stellaaaaaaaaa! where you help her build a sports car. After you finish the quest, you will often see her driving by your town like other visitors in her red sports car. If stopped, she will usually spend around two hundred or three hundred coins, 1-3 times in the same shop (so 200 to 900 total). She will spend much more during a Fiesta.
"Hi, I am Stella! I am sad... My car had flat tire not far from your beautiful village. Can strong and kind boy help me change tire?"
You will only see Stella driving down your street after you finish her quest in her red convertible, similar to Kelly's.