You've mistaken Summer for Midge. Midge was Barbie's first ever (best) friend. She was introduced in the '60s and disappeared and reappeared a couple of times. It's assumable that Mattel has dropped her off their doll line. And yeah, she was replaced kwa Teresa.
Teresa was Barbie's friend much before Summer appeared.
Summer doll appeared in 2004 as part of Californian beach, pwani collection, AFAIR. As for roles in BMs, Aramina is her best-known role (she also played Luci in IP, Melody in DC and Taylor in FS). Hope this will help (a little).