Annie's relationship with her mother is arguably the most important relationship in the book. At the start of the story, Annie is as close to her mom as any child could be. The whole world revolves around her; she is the sun, the moon, and the stars. They are more like best pals than mother and daughter, and they are always doing something together, whether that means going clothes shopping or just hanging out. They really are inseparable.
Then, Annie hits puberty. Her relationship with her mother changes dramatically; her whole outlook on relationships changes too. When Annie starts menstruating, her mother backs off immediately. Sure, she has the best of intentions; she wants Annie to put away childish things and start behaving like a grown-up. Annie does not understand. She just does not understand why her mom is so distant all of a sudden. Mom does not even want her to dress the same way as she does anymore. It is all rather bewildering for Annie. Her whole identity has always been intimately linked to her mother, but now she has to start over by constructing a totally new identity from scratch. Adolescence is supposed to be hard, but this is painful.
This one relationship is seemingly gone forever. What now? Annie seizes the opportunity to establish some new relationships. Her new school is just the place to do it. Her new school allows her to foster new relationships. It all makes perfect sense. However, new beginnings in this story are also associated with death. Annie does not just hate her mom now, she also wishes she were dead. Already obsessed with dead bodies in coffins, she now fantasizes about her mom being one of them. The death of one relationship leads to the birth of another. In her end there is also a new beginning.
The intense, passionate love that Annie once had for her mom is now transferred to Gwen, her new best friend. To any dispassionate observer it is blindingly obvious that Annie is on the rebound from the break-up with her mom. Annie is blissfully unaware of all this; she has fallen for Gwen in a big way. Still traumatized by the sudden split with her mom, she is desperately trying to get back that all-consuming love she hoped would last forever.
Unfortunately, Gwen cannot act as a mother substitute for very long. For one thing, she is not very mature and soon starts to annoy Annie with her childish ways. What Annie fervently hoped was a new, large helping of mother love turns out to be just another girlish infatuation.
It is at this point that Annie realizes that she cannot ever hope to reclaim that extra special feeling of love and security she once had with her mom. She may well have broken up with her mom, but in her brief platonic friendship with Gwen she still tried to hang on to some of the things that characterized their seemingly unbreakable bond. Once things with Gwen turn sour, she rejects everything associated with her mom. Mom is dead to her now.
As a result, it is high time for a little juvenile rebellion. What better person to guide Annie than the mysterious Red Girl, an uber cool tomboy who can climb trees better than any boy, plays marbles, and loves sneaking into abandoned buildings. If Gwen was a surrogate mother for Annie, the Red Girl is a kind of anti-mom, someone who represents everything her mom hates. Love doesn't really enter into the equation here; Annie's connection with the Red Girl is a relationship of convenience, nothing more. Ultimately, it is Mom who puts an end to Annie's brief acquaintance with the Red Girl, but it would've fizzled out before long anyway. Of that we can be reasonably sure.
We do not have to play at being amateur psychologists to realize that the relationship between Annie and her mom has hampered her ability to form meaningful relationships of her own. Ironically, this means that Annie is still deeply connected to her mother, despite the formal split between them. So long as she cannot establish strong connections with other people, she will always in some way remain tied to her mom's apron strings. Unless she can truly break free, she will never be a person in her own right. She will forever be defined by not being her mother, rather than being Annie John.