We can best sum up Hiroko Ai as an enigma—which is really just a fancy way of saying we know next to nothing about her. It sounds better, though, right? Plus, we didn't even come up with this label by ourselves. Maya calls her an "enigma" (2.3.116) early in the novel and the label just kind of sticks.
Hiroko is the main agriculturalist on the Ares, but she spends most of her space-hopping days on the farm surrounded by her cronies. Once on Mars, she and her peeps once against stay a tight-knit group until they simply up and vanish one day, taking the group psychiatrist, Michel, with them.
By themselves, they start an isolated Martian colony, separate from the rest of humanity's endeavors. She pops up every now and then throughout the novel, and in the end she opens her house to the First Hundred who survive the Martian revolution.
Although we can't say much about Hiroko, it is obvious that she has a personal vision for what Mars can become. She's also a type of thematic glue in the novel. Just take a look at our "Symbolism, Imagery, Allegory" section, and you'll notice that every single one of the entries there can be linked to Hiroko in some way:
Hiroko ties these and other symbols and themes together, but the darnedest thing is we can't say why. Maybe her character is meant to hint at an answer the First Hundred don't see but that's right in front of them the whole time. She could be a way for the novel to tie the all the themes together into a spiritual or religious package. Maybe her character is simply set up for the sequel.
Like we said, enigmatic.
This isn't to say that there aren't a few helpful clues sprinkled through the novel about Hiroko, though.
John refers to Hiroko as their "Persephone" twice in the novel (5.8.143; 5.10.105). Persephone is the daughter of Demeter, goddess of agriculture, and the wife of Hades, god of the underworld. Persephone spends six months of the year with her mother on the surface, while the other six months are spent with Hades in the underworld. Her comings and goings result in the seasons we see during the year.
This connection to Persephone links to Hiroko's relationship with the other First Hundred. She brings life via agriculture to Underhill—but then leaves. She returns to help John Boone solve his mystery—and then leaves. Her final appearance is to welcome the surviving First Hundred to a new life in her home. And then she… well the book ends so we really don't know. But we wouldn't be surprised at all to find out she bounces.
Oh, and did we mention "Ai" translates to love in Japanese? It really does, but there's something you should know. "Ai" is the Japanese word for the general emotion of love, whereas "koi" is the Japanese word for love of the opposite sex. In other words, koi is a love that wants, while ai is a pure love that gives.
Although these references hardly solve the mystery of the character that is Hiroko, we feel they put her mystery into perspective. She may always be leaving, but she's also always giving as she goes.