I unfortunately haven't developed a quirrellmort yet (the concept is on my to-do list though, along with a number of other personifications). I do have two loose internal models though, for very specific tasks.

The first is called "The Alien" or just "Alien". I created it in my mid-teens after reading the last samurai (not the movie), although my use of The Alien is not the same as the book's. The Alien is the voice in my head that says the pointlessly stupid or cruel things (generally about people) for no reason other than being able to. They aren't things I actually believe or feel, so I just tell The Alien to shut up. By doing this, I can create a divide between myself and these thoughts, not feel guilty about them occuring, and more quickly put them out of my mind.

The second I created very recently based off this thread. It is for the prevention of ego depletion when it comes to either starting big tasks or taking care of long lists of little tasks. Rather than think "Ok time to (make myself) do this" I defer the choice to an internal, slightly more rational model of myself that doesn't suffer from decision fatigue. The outcome is very predictable ("Do the goddarn task already"), but does seem to work very well for me. It's still quite new, and I probably don't use it as much as I should.

I have plans to make a number of other internal models to create an internal 'parliment' that can discuss and debate major decisions, or act on their own for specific required benefits. Other models that might be included include a cynic/pessimist (to help me be more pessimistic in my planning), an altruist (to consider if my actions are actually beneficial), a highly motivated being (to help renew my resolve), and some kind of quirrellmort. These are probably very liable to change as I try to implement them.

He mostly tells me to kill annoying people.

Do you have more than one such useful sub-personality?

No, but I'm working on them. I've found my inner Hufflepuff to be particularly helpful in actually getting things done.

Incidentally, is there a name for the "sub-personality technique?"

You know that spreading rationality is a strong net positive, right? How many lives could we save if people just stopped for a while and though about stuff in a relatively unbiased way? Even then the population of purely selfish but rational agents could do better than we do - and people usually aren't purely selfish. If we could only spread rationality better. But you know as good as I do: it's exactly the biases that make demagogy almost always sounds more convincing than the truth. It is so hard, so frustrating to explain the bitter truth, while competing against comforting lies, pushing all the buttons that - you've learned it - almost guaranteed to make one agree.

But what if you could do a little bit of... you know... marketing? Oh, spreading rationality through irrationality sounds so hypocritical!.. deontologically. But you're utilitarian, you know how to make trade-offs. And you know better than to make trade-offs against some general principles that may be reasonable rules of thumb, but don't even start to encompass the actual people and their happiness. How did you put that - shut up and multiply? Well, go on, multiply: billions of lives saved against millions slightly offended. And here is the thing - before learning about biases they won't be able to recognize your little tricks, and the job would be already done. Many will probably agree that it would have been net positive. Oh, your reputation could be damaged? Well, I though you were an altruist.

Can it even get any worse than it is now? I'm not even talking about the marketing of commodities - adding a little bit of your marketing isn't gonna change anything at all, even if you still believe in those deontological ideas. I'm talking about the market of ideas. You compete against people who learned some of the tricks, but use them with malicious intents, not for the benefit of the consumer. But you know better. They vaguely learned some buttons from classical novels and books by liberal arts majors. You learned how the whole machine works, with mathematical modeling. You know what buttons to push to make your point sweeter ans stickier. You can crush all that irrationality all at once.

After all, there are no arguments without any flavor with them. It's just that you either select to give the randomness and subconsciousness to choose the flavor, and call it "fair", or purposefully select the flavor, and call it "trickery" and "marketing". But since when do rationalists consider obliviousness better than knowledge?

Why do you choose to not use your force for good? What stops you? What's your choice?

I've often considered producing such a personality, after observing a previous LW discussion about tulpas, but never even got past the stage of which character to use - I don't know who the "most competent person I can imagine" would be.