Learn English – Word for a forgetful person

In standard English usage, fellow is neutral and informal, as Jim notes in his comment. It would be appropriate for relaxed gatherings. Think of a speaker at a technology conference, for instance:

"And I tell you, this fellow right here, he is an inspiration to all of us."

It would also be appropriate at parties. Think of the song:

"For he's a jolly good fellow."

On the other hand, it can be used negatively as appropriate. In these contexts it would indicate derision or intensity.

"Nasty fellow, always coming up with some new scheme for getting his own money with other peoples' effort."

"I won't deny, he works like a horse, but nice? No, he's definitely not a nice fellow."

The use of the word should be a deliberate choice if the context is not completely casual. In a small group of friends having drinks together, it's pretty much always appropriate. A speaker might use it to help loosen up his audience, as in the first sentence I noted. Using it in writing is inadvisable unless the context specfically calls for informality and a folksy feel. A find/replace from person to fellow is not a safe or good idea.

Firstly I will take some liberties and make some presumptions as to what your are really trying to say.

I am trying to learn the violin and so sometimes I play the violin. In this case, can I call myself, "who’s is now playing a violin", a “violinist”?

Well technically yes, because according to my dictionary; violinist; a person who plays a violin

But most Truck drivers would not call themselves a Trucker until they passed their test and were employed driving a truck. Most people who are learning or not accomplished in a particular skill would acknowledge that fact and term themselves:

Apprentice, Student, beginner, learner, amateur or something similar.

violinist C.E.D.noun [ C ] UK a person who plays a violin