Isaac Caspar Sachs meets Luisa Rey by accident (or is it fate?) when she sneaks into Dr. Sixsmith's office and Sachs just happens to be raiding the place. Like Sixsmith, Sachs is suffering from a chronic bout of conscience. See, he was part of the Seaboard Corp. cover-up of the dangers of the Swannekke reactor, and now he's feeling kind of guilty about it. He tells Luisa that he's "[t]oo cowardly to be a warrior, but not enough of a coward to lie down and roll over like a good doggy" (3.31.1).
Sachs wises up about it, though, as his middle name would seem to suggest. (Caspar is one of the three Biblical Wise Men.) However, instead of frankincense or myrrh, he gives Luisa something useful: Sixsmith's report which exposes Seaboard's cover up. Oh yeah, he kind of falls in love with her, too. But remember that "too cowardly to be a warrior" part? He's still letting her do all the hard work. Some gentleman.
Even though he's not a warrior, Sachs still gets slain by Seaboard. He's aboard the plane that blows up, which conveniently takes him out of the equation. Not only that, but the report he gave to Luisa sinks in the ocean after Smoke runs her off a bridge. Sadly, Sachs's sacrifice (Sach-rifice?) seems to have been for nothing.