Wintermute is an AI built by the Tessier-Ashpool company. It's also the figure responsible for putting together Armitage's team and planning the heist. It exists for one purpose and one purpose only: to merge with his other half, Neuromancer.
Why? That's an excellent question, and one even it can't answer. See, the only reason it can come up with is compulsion. It doesn't know why, it just has to. The urge is part of its programming (17.46).
Personality wise, Wintermute doesn't have much of one. That means that when it communicates with anyone, it has to take on the form, mannerisms, and speech patterns of someone from that person's past. For example, when it talks to Case, it often talks to him in the form of the Finn, but sometimes Julius Deane or Lonny Zone.
The one thing we can say about Wintermute's personality is that this thing is seriously calculating. It can create intricate plans, and profile people to find the perfect pawns for said plans. For example, Wintermute had a key hidden in Straylight twenty years before Molly needed to use it (15.28). Plus, it had the boy killed, so you might call it cold, too (from a human's perspective).
The key here lies in the fact that Wintermute is an AI—artificial intelligence. As in, not "artificial emotions." It can fake feelings if it's taking on the form of another human, but it most certainly doesn't have any itself. It's just got its goal (the Neuromancer merger), and the rest is riff-raff.
On the other hand, Neuromancer has personality. A bunch of it, in fact: "personality is [its] medium" (23.54). That might sound deep and abstract, but they're actually talking computers here.
How's that? Well, Case says that Neuromancer is full of RAM (22.50). RAM stands for random access memory, and in this novel, it's the opposite of read-only memory, or ROM. So in order to understand Neuromancer, we need to stack it up against a figure in the novel that's full of ROM. That, ladies and gentlemen, would be Dixie.
As ROM, Dixie can't grow or obtain new memories. The ROM construct consists only of the data that Dixie is—or rather was. It can never change. Game over; never pass "Go" again, never collect $200. Neuromancer, however, can change, can grow, and can obtain new memories because it's full of RAM. It's not chained to any data set and can alter itself whenever it pleases. It does just that when it brings Linda Lee into its world, changing itself by writing her data into its own. So when it says, "I call up the dead," it kind of means it (21.31). (That's also, by the way, a play on the word necromancer, which is a magician who can commune with the dead.)
So in many ways, the Neuromancer, with its excess of personality, is the opposite of Wintermute. They're like the yin and yang of cyberspace. There's just one problem: Neuromancer doesn't want to merge with Wintermute at all. It tries to simultaneously kill/bribe Case with a chance to live with Linda Lee just so Case won't finish the job he's been hired to do. Why it doesn't want to merge with Wintermute is just a big a question as why Wintermute wants to merge so badly with Neuromancer. Programming, we guess.
P.S. If you're curious as to what the name Neuromancer means and why he's the title character, check out our "What's Up With the Title?" section for more discussion.
Um, good question. It's not really clear what becomes of either Wintermute or Neuromancer by the novel's end. All we know for certain is that the two merge as per Wintermute's desire (so we guess he wins on that front), and neither technically exist anymore because, having merged, they're something new altogether. They're both now the entirety of the matrix. So what's changed? What has become of the matrix? Shmoop amongst yourselves.
And for more on Wintermute and Neuromancer's merger, check out our "What's up with the Ending?" section.