Eti-OSA is the one you think is nerdy; she's beautiful even if she doesn't know it. She is incredibly gifted and is always willing to help someone out. She will listen to your problems and try to help you as much as possible. Everyone wants to be her friend. She is loyal. She is incredibly beautiful and full of grace.
A commonly-used term in romantic anime, an osananajimi is a friend from infancy, a childhood friend or old playmate with whom one shares the special and intimate understanding that (according to such stories) can only come from having known each other since earliest childhood.
A good example of Osananajimi is Manami Tamura from Ore no imouto