I am assuming from your username and the language that you use that you are a fellow user of .info or /r/tulpas. Funny coincidence for such a small community.
I'll go ahead and propose my own alternate ideas and theories of what tulpa are, as I think they are a bit more solid and somewhat more digestible to the skeptical onlooker.
The average person goes through their life with many assumptions. The sun will come up. They open their eyes to see. Punching someone will hurt your fist. These assumptions are very built into the mind, and determine as much of what we see as the physical world around us. The mind auto-corrects and fucks with every input we get from our senses every day, most often without our knowledge.
For most all human beings, raised from birth being called a name, taught to see other objects as other things, and so on, one of those assumptions is "I am a single person".
When you think of it, on a deeper level, such an identity is a very arbitrary and odd thing to say. We aren't "a single person", we are a collection of thousands if not millions of systems working together to form an emergent trait that is "us". To call that thing "a single person" can very well change.
So, where there is a possibility, people have tried, learned, and found that these assumptions can actually be changed, and there is a somewhat repeatable process that produces said change.
So you do this process, you take those steps, and "you" can become "me and my tulpa" in the sense of there being another, smaller, being in your mind. It can also become "me and my tulpa" in the sense of two equal beings within one mind, or more.
Now, you have this new assumption. You change your view of the world to be that "I am not just me". And by doing it in the "proper way", and not having this caused by trauma, anger, annoyance, or just a plain lack of control over your own thoughts, you are a happy, healthy, and functional human being(s).
So, now, the brain has to think differently. Before, there were internal and external thoughts, but now it has to take thoughts and add a new dimension. Internal, external, and "who is this thinking this?".
So it does, it looks at thoughts and learns to arrange and screw with them in a way that produces the effect of many speaking and active entities in one mind. Some have the opinion that their tulpas are conscious all the time, and they experience that as reality. Most have a tulpa in the background, some little thing they can address and speak to at any time, and it is there, and they experience that as reality.
So this isn't truly a practice of making some new person in your head. It is still one train of thought, just twisted into curves so that if you line the pictures up correctly it looks like two trains at once. There aren't two separate thinking beings in anyone's head, as it's actually impossible to accomplish such a thing. Humans do not multitask, as is proven in many, many, studies.
It is a practice of changing your biases through a process and community. Very similar to how people believe in things like exorcisms, and INCREDIBLY interesting to note that such a unstudied and unrealized process can be discovered and used in such a way.
So is it ethical? Yes, so long as it doesn't result in a net harm to society, and I would argue it is a net benefit.