Hae-Joo is the Unionman who shows Sonmi what it means to be alive. She might know about life from having her nose in almost every book (or the digital equivalent of one) under the sun, but she doesn't know how to live it. Hae-Joo changes this by taking her shopping, showing her how both upper- and lower-class people live, and revealing top government secrets, like the fact that fabricants are murdered and recycled for food. Wow, that's a heck of a first date.
Okay, let's back up a bit. Hae-Joo is sympathetic to Sonmi and the Abolitionist cause from the moment we meet him. He tells Boom-Sook that Wing027 has been killed, and while Boom-Sook laughs, "Hae-Joo then did an unusual action; he looked at [Sonmi]" (5.1.181). This is startling for her, because she's used to receiving about as much attention as an old vacuum cleaner in the corner.
When Boom-Sook is removed from the picture, Sonmi is put into the care of Hae-Joo, who takes her on the aforementioned whirlwind tour of the world. Along the way, he shows her all sorts of contradictory images (Seer Kwon brutally murdering a fabricant in front of a beautiful river, for instance), and Sonmi doesn't quite know how to process them. Hae-Joo tells her that there is no way to rationalize the vileness of human behavior with the sublime beauty of nature: "These... xistential qualms you suffer, they just mean you're truly human. [...] You don't remedy them. You live thru them" (5.1.353, 5.1.355).
At the end of her orison, Sonmi tells us that she believes Unanimity orchestrated everything. Even Union, the supposed rebels, are manipulated by Unanimity to stoke society's fear of fabricants. If this is true, does that mean Hae-Joo was in on it too? Did he treat Sonmi like a tool until the end, or did he truly care about her? If he cared about her, why did he let her die?