Unbreakable character

Known for his unbreakable character and resilience, Stockdale described the two defining characteristics of American captives who were most likely to survive the brutal conditions of a Vietnamese prison. First, they openly faced and accepted rather than ignored or dismissed the harsh facts of their difficulty. Second, they never stopped believing that they would someday get out. (source: an article written by a non native speaker)
I googled the meaning of the underlined expression but could not find any.Is the underlined expression common? If so, what does the underlined phrase "unbreakable character" mean? Thanks.