Barbie's little sister Skipper saw the light of day in 1963. She is probably meant to be around nine years old with a flat chested torso. Had blue eyes in a side glance but hair in the colors blonde, brunette or titian. Just as Barbie and her friends Skipper came with stiff legs to begin with, but got the bendable ones at the same time as her big sister.
Growing Up Skipper was also made with this head but her body is only partly made from the original body. She got a soft torso with a growing mechanism inside. You turned one of her arms and she grew half an inch and tiny breasts developed. Turning the arm back the other way, and she shrunk and the breasts disappeared again.
A couple of Skipper's friends was made with this head, as seen below.
From what I have learned, the early Skipper dolls had no visible markings, neither in neck nor inside rim. That came with later dolls, probably at the same wime the Bendable Legs (AG) Barbie came along.

Skipper
Copyright: 1963

Skipper

Straight Leg Skipper, 1964

#1069 Sun Set Malibu Skipper, 1971-1977

#9926 Partytime Skipper, 1976
(Europe)

Funtime Skipper
(Europe)

Great Shape Skipper

#2854 Music Lovin' Skipper, 1985
(Europe)

#BCP79 50th Anniversary Skipper, 2013
blond version (there is also a brunette repro)

Japan Skipper

The Japanese Skipper dolls looked slightly different than the western versions with their dark, close to black eyes and side glance.

# Japan Skipper
Doll belongs to/photo taken by and used by permission from Gerry

Japan Skipper
Doll belongs to/photo taken by and used by permission from Gerry

Japan Skipper
Doll belongs to/photo taken by and used by permission from Russell Gandy

Ginger

When Skipper grew the first time she got a new friend also - only seen in that version. Ginger shared Skipper's head sculpt as well as the growing mechanism in the torso. Ginger has brown eyes and hair, otherwise she and Skipper are very much alike, or actually identical :) 

#9222 Growing Up Ginger, 1975

Skooter

One version of Skooter was sold in Europe, in which she was made with Skipper's head and not her own. She has been one of my holy grails but eventually I managed to get my hands on a played with doll, still in her original blue two part swimsuit. Funtime Skooter has teal eyes and dark red hair.

#7981 Funtime Skooter 1976
(Europe)