I cannot claim to have ever seen a demon. However, I have encountered plenty flesh beings that sure act as if demons inhabit their flesh. So I would not begin to know what you are describing, as in having a first hand visual.
How old is this child? Is this the same house and same window that your brother’s experience took place?
The child is 4 and no it’s a different house....totally far removed from where my brother has seen it.
In fact, it’s the first time my brother has told me of it, or spoke of it. We were talking about our childhood and how and why things have happened.
He told me this because he thinks there might be something going against us, on purpose. We just don’t know what it may be.