You are an Envelope
envelope
Only the letter N.
No other letter begins with that letter. The only letter that begins with it is W.
"EMIGRATE"
"UPWAFTED"
nutria
Installs - however this is eight letters long
No. There are at least 29,302 words in the English language that have eight letters.
The only single-word eight-letter anagram of these letters is athletes.
I can only think of halitosis [but that is 9 letters]
Nauru, Nepal, and Niger are countries. They are the only countries that contain only 5 letters and begin with the letter N.
No country begins with the letters Qu. Qatar is the only country that begins with the letter Q. It is in Asia.