Franchise context: The write-up below is from the parent franchise article (anime, book series, or IP) rather than the card game itself. A dedicated CCG profile has not been written yet.
A flagship is a vessel used by the commanding officer of a group of naval ships, characteristically a flag officer entitled by custom to fly a distinguishing flag. Used more loosely, it is the lead ship in a fleet of vessels, typically the first, largest, fastest, most heavily armed, or best known.
Source: en.wikipedia.org/wiki/Flagship