For example there is "observer" for somebody who professionaly follows a subject. What about awareness? (I mean a word in the meaning of imaginary word "awareness+er")
An afficionado is a person who likes, knows about, and appreciates a usually fervently pursued interest or activity.
An amateur is one who engages in a pursuit, study, science, or sport as a pastime rather than as a profession.
An enthusiast is one who is ardently attached to a cause, object, or pursuit, or one who tends to become ardently absorbed in an interest.
An expert is a person who has special skill or knowledge relating to a particular subject.
And a maven is one who is experienced or knowledgeable.
In my opinion the word "maven" is less usual in formal writing than the others.
If you follow those links you will find some sample uses of these words.
"Observer" is not at all a normal word used to describe someone who professionally follows a subject. Normally this would be an "expert." However, specific fields often have jargon to describe their professionals. For example: watchdog, commentator, analyst.
Your question about awareness is less clear. Do you mean someone who follows a subject but not at a professional level? Again it depends on what the subject is. Some candidates are: fan, follower, enthusiast, watcher.