Most people have several different names. Here's an example. My grandfather, who had the Hebrew name "Yitzhak". His Yiddish name was "Itzik" (spelled "Icek" in Polish), the Yiddish version of "Yitzhak". After immigration to America, he was called "Jack" by co-workers, because they couldn't pronounce "Yitzhak". "Yitzhak" became "Izzy" among family, and he was naturalized as "Isidore".

(Note that despite the fact that the English language version of the Hebrew name "Yitzhak" is "Isaac", he was never "Isaac").

So which one is his "real" name? -- They all are. There's no such thing as a "real" name. A name is what people call you. Often the name on some official document is one that was never actually used by that person during their lifetime.