Aech is Parzival's best friend inside the OASIS, and Wade's best friend even though the two have never met in real life. Common interests—movies, music, and video games—and a common goal—the quest for Halliday's egg—made them best buds right off the bat, showing that friendship is possible online. In fact, it's even less judgmental than friendship in the real world.
We see Aech through her avatar, "a tall, broad shouldered Caucasian male with dark hair and brown eyes" (3.6). Yes, we said "her." More on this in a minute. We also know that she makes money through pro gaming, something that's gaining in popularity even today.
Other than that, though, we don't get to know too much about Aech. Despite being BFFs, Wade lets the relationship fall by the wayside when he starts hanging out with Art3mis, and he puts the Hunt of Halliday's egg higher on his priority list than this alleged best friendship as well. Because of these two obsessions—girls and game—Aech is absent for a significant part of the book, until the plot brings them together. In person. And Wade is in for quite a shock…
When Aech picks up Wade in her awesome solar-powered RV (that's right, she lives in an RV. Pretty awesome. You've heard of Hell on Wheels, but this is Helen on Wheels) she finally reveals her true identity to Wade: Helen Harris. Helen is so different in real life from her avatar that it's almost comical. The world of Ready Player One is practically devoid of minorities, so it's up to Aech to bear the brunt of them all: African-American, female, and lesbian. All in one package. Thankfully, Wade realizes that "the young woman sitting in front of [him] was [his] best friend Aech" (33.51). Who cares what she looks like when he knows she has his back?
This unexpected development shows us Wade's tolerance and that the depth of their friendship transcends her deception and differences. Wade still considers Aech his best friend, even though she's the opposite of everything Wade thought she was. But he still insists on calling her "he" despite this, so we're not quite sure what to make of that.
Helen never says she identifies as male, genderwise, just that it was easier for her, as a black woman, to create an avatar that conformed with society's majority: white, heterosexual male. It's 2045 and we're still dealing with racism and sexism. It's going to be hard to change society's prejudices if minorities must always pretend to be the majority.