'Nuff said. Great profile, nice smile and blue bikini make for an attractive portrait. The shapes are nice and clean, the detail on the hair works for me, and the blue / pink combination is pleasing.
Also of note - this is perhaps the first portrait where the open-mouth detail (teeth, lips and gums) has been given more thought - most of the following portraits follow a similar structure in this area, and in this case it adds a lot to the expression.