What role does Athena play in The Odyssey and why is she often called its main female character?

Athena is
the goddess of wisdom, as well as a multitude of other things, but it is this wisdom that makes
her the ideal character to truly play opposite Odysseus.  While Penelope is stuck home, waiting
for her husband's return (which might never happen, but she is faithful and continues to wait
for him), it is Athena who guides and helps Odysseus throughout his long journey home to his
wife and son.

Odysseus is known as a trickster, a clever man, even back in his
days in "The ," where, in addition to many other examples, he was the one who
concocted the idea of using the Trojan Horse to get into the walls of Troy.  As you'll see in
the link below, some scholars feel thathad to use an immortal, and a very wise immortal at that,
to be the "leading lady" of this story.  Homer wanted to highlight the wisdom and
cleverness of Odysseus, and so he chose the goddess of wisdom to help him through his journeys. 
He also wanted to illustrate the fact that Odysseus was a worthy person - the gods and goddesses
didn't just help any old person on the street - they only interfered in the lives of mortals who
were deserving of the help.

Check the link below for more information about
Athena and the other characters of this story.  Good luck!