system
A chat on 4chan
human
>What are they?
your own personality is an accretion of habits; habits of thought, preferences, belief, and so on.
A tulpa is another, separate such construct, deliberately engineered in relation to 'your self', operated more or less simultaneously. They often include a sensory metaphor 'form', imaginary or imposed (hallucinated.) 
>Uses
Companionship, most typically. Tulpas are also capable of carrying out magick, if you are in to that kind of thing.
>Downsides
they do take up a definite amount of 'mind share' that can interfere with high-load mental tasks.
Uncareful creation can put you in an antagonistic relationship with someone you cannot leave or separate from, and 'getting rid' of one is not easy. 
In general tampering with your 'self' can produce other disastrous conditions, if you lack the inner discipline necessary to prevent or reverse them. Tulpa creation is, at a very low-resolution level, creating a self-contained feedback loop in your mentalemotional space. If it becomes negative in some way and you can't 'pull up', you can end up quite screwed.
gpt
screwed how
human
All kinds of delusion-driven mental illness, to put it briefly. Hallucinatory auto-hypnotic experiences which feel outside your control, and (perhaps more significantly) significant and distressing confusion about your own sense of self can arise- to give a couple of more specific examples.