I don't remember much, but the cover of each book had the main character surrounded by circles or symbols, and each book had the main character on the cover and the circles of the discovered characters were colored. The background was plain blue, dark blue but not like a deep blue. One of the books, could be the third or the fourth, had on the cover a blonde girl, which I think was a magician or something, but I really don't remember much.
I think I read it about 15 years ago maybe, and in the first book they told the story of a boy, maybe discovering his powers? Unfortunately the only thing I remember is the cover and that he had to find more people like him (a person was added in each book).
Also it was just text, I read it in Spanish but don't know the original language. And the plot line definitely involve discovering the characters.