Achilles was a famous Greek HERO from Homer's Iliad, known for his strength and courage in battle. He was a MORTAL, as indicated by the 6-letter solution to the crossword clue. He was also a HERO, as indicated by the 4-letter solution to the crossword clue. Finally, Achilles is most famously known for his vulnerable heel, which was caused by his mother dipping him in the river Styx when he was an infant, leaving his heel unprotected. This is referenced in the 6-letter solution to the crossword clue, which refers to his TENDON.