Final answer:
A heroic couplet must consist of two rhyming lines written in iambic pentameter, with each line containing ten syllables. It must not necessarily include legendary figures or have influenced later poetry, but it often has a complete grammatical structure.
Step-by-step explanation:
The characteristics that a couplet must have to be a heroic couplet are specific. First, a heroic couplet must consist of two lines that rhyme. Secondly, it must be written in iambic pentameter, which means each line has ten syllables with an unstressed syllable followed by a stressed syllable, for a total of five metrical feet. Finally, although a couplet may have a complete grammatical structure, it is not a necessity for it to be considered heroic. This style of poetry became famous through the works of poets such as Alexander Pope. For example, in Pope's 'An Essay on Criticism,' he uses rhyming couplets that are characteristic of the eighteenth-century heroic style, reinforcing classical qualities of balance and harmony.