Native speakers of a language are fluent in their dialect of that language. The average native speaker is in fact the standard by which fluency is measured.
It is possible to be a native speaker of more than one language, although the term implies that one has learned all such languages extremely early in life. One's phonemic grid begins to form at least as early as the first attempts at babbling, and it becomes much harder to learn new languages somewhere around puberty. However, many people are native speakers of more than one language because they learned to use those languages in childhood. One parent might speak only French to a child, while the other speaks only English.