All of Robert Frobisher's letters are addressed "Dear Sixsmith," to which we respond, "What's a Sixsmith?" It turns out Rufus Sixsmith is Robert Frobisher's friend, lover, and enabler. Wait a minute? Enabler? You read that right. Frobisher wouldn't be have succeeded at half his schemes were it not for Sixsmith's forged letters, loans, and mediation with the shady book buyer Otto Jansch. Sixsmith tries to "save" Frobisher, looking for him after his second-to-last desperate letter. Alas, he is too late. While the argument could be made that he inadvertently helps Frobisher along to his inevitable demise (after all, apathy vs. action is a big theme of the book), but he was blinded by love, so how can we not forgive him?
Perhaps Sixsmith privately feels the same way we do. This apathy vs. action theme becomes a recurring one when he reappears in Part 3, The First Luisa Rey Mystery (Sixsmith is the only character to appear alive in more than one time period). In the 1970s, Sixsmith is having a similar dilemma, but this one is work-related, not love-related. He doesn't know whether he should risk his life and go public about a potential nuclear disaster or keep it a secret.
Sixsmith karmically redeems himself for standing by while Frobisher self-destructed when he supplies Luisa Rey with his report. This moment is all the more bittersweet when you consider that Luisa Rey might just be Frobisher reincarnated: Sixsmith says to her, "I feel I've known you for years, not ninety minutes" (3.6.15), and after all, she has the same comet-shaped birthmark Frobisher had. Add to this the fact that Frobisher was always suggesting that Sixsmith "try 'em [him]self one time, women I mean" (2.5.3).
But Sixsmith can't consider Luisa in this way. One, he's still lost in love with Frobisher, reading his letters again and again: "[Sixsmith] witnesses himself through Robert's words searching Bruges for his unstable friend, first love, and if I'm honest, my last" (3.17.2). And two, he's killed too soon, paying for his action with his life.