As I mentioned in the topic, I'm confused about how to pronounce "an unreliable person" in a native way, because when I pronounce them seperately, such as an un, I think it sounds a little bit redundant. Should I mix them into one sound, sounds like an reliable, or pronounce like a nunreliable or just speak them normally without any alteration?
The short answer is: do not say "an reliable"; do say "anunreliable." Don't worry about it sounding like "a nunreliable." This happens all the time in English. Often there's no problem at all, because "nunreliable" isn't a word, and even if it were, the context would probably make the meaning clear.
You're not the first person to have this problem. In fact, so many English speakers have made these kinds of mistakes - which we call rebracketing or misdivision - that common words have become permanently transformed.
For example, the word a notch was originally an otch, a newt was once an ewte and a nickname was an ekename.
In late Middle English a and an commonly were joined to the following noun, if that word began with a vowel, which caused confusion over how such words ought to be divided when written separately...
Other examples of this from Middle English manuscripts include a neilond ("an island," early 13c.), a narawe ("an arrow," c. 1400), a nox ("an ox," c. 1400), a noke ("an oak," early 15c.), a nappyle ("an apple," early 15c.), a negge ("an egg," 15c.), a nynche ("an inch," c. 1400). A manuscript from c. 1500 has a nylle for "an isle."
The reverse also happened - more frequently in fact - where words that originally had an n lost them:
a napron -> an apron
a nauger -> an auger
a nadder -> an adder
a noumpere -> an oumpere (umpire)
The point is, even for native English speakers, when speaking quickly, there's usually no clear boundary between "an" and a word that begins with a vowel. When speaking slowly, you should include a slight pause between the words.
I agree with Weather Vane, another word for the concept is 'flake,' though, if you wanted a synonym. You would use it like "That person is a flake."