The Siren of OASIS
Kira Morrow, née Underwood, died in a car crash ten years before the events of the book get rolling, but without her, the whole book wouldn't have happened. She's the reason why Halliday's contest exists in the first place. When she married Ogden Morrow instead of Halliday, he never got over it, becoming a crazy recluse who used her D&D character's name, Leucosia, as his password all the way until his death… over fifty years later. She must have been quite the catch.
Here's the thing: the book never actually shows us why Halliday pined for her for half a century. But it's telling that her alias, Leucosia, is a siren from Greek mythology. We guess she just had a supernatural allure. All we know is that Halliday had a crush on her because she was the only girl in his gaming group. She also gave him the nickname Anorak, and when someone names you after an ugly jacket, well, it must be love at first sight, right?
At the end of the book, we see that Kira was buried on Ogden Morrow's property, kind of like Elvis being buried at Graceland. Wade glimpses her tombstone, but can't make out the epitaph. We're never told the epitaph either, so it remains a mystery, like the woman herself.