Athena and Poseidon didn't have a great relationship(which frankly wasn't unusual for the Olympians). They were rivals. One example of their rivalry was their fight over Athens. Both of them wanted to be the patron god of the new city.
Athena and Poseidon entered into a contest to be the patron of Athens. Poseidon produces a spring of water but it was salty. Athena bested Poseidon by producing an olive tree on the Acropolis. Poseidon also raped Medusa—a mortal who had the reputation of being beautiful—in Athena's temple, desecrating it.
Athena won the contest and became the patron goddess of Athens. From that time forward, Poseidon and Athena were rivals. This plays out in the story of the Odyssey where Poseidon tries to thwart Odysseus while Athena tries to help him on his journey.
ATHENE (Athena) The goddess of warcraft was, according to some, the daughter of Poseidon and Tritonis (contrary to the usual account in which she springs fully-grown from the head of Zeus).
Gods play an important role in Homer's epic the Odyssey. Almost all the gods are sympathetic to the main hero, Odysseus, except for Poseidon. Poseidon hates Odysseus because he has blinded his son Polyphemus.
According to legend, Athena became the patron goddess of the city of Athens after a competition with Poseidon, though he remained on the Acropolis in the form of his surrogate, Erechtheus. After the fight, Poseidon sent a monstrous flood to the Attic plain to punish the Athenians for not choosing him.
Not satisfied with lordship over the sea, Poseidon coveted earthly realms as well. In his dispute with Athena for dominion over Athens, the two gods had a contest as to which one could give the Athenians the best gift. Poseidon shoved his trident into the Acropolis and produced a flowing stream or a horse.
On another occasion, Poseidon seduced the gorgeous maiden, Medusa, in one of Athena's temples. Naturally, the virgin goddess was enraged that her temple was so defiled. Athena retaliated by transforming Medusa into a hideous monster with snakes for hair who would turn anyone who looked at her into stone.
Enemies: Athena's most famous enemy was Medusa, a gorgon with snakes for hair who could turn people to stone with her gaze.
Poseidon's biggest enemy would be Athena. There are two main reasons. One: Poseidon and Athena competed to become patron god/goddess of Athens. Athena created an olive tree and Poseidon created a salt water spring.
She never had a true lover or someone to hug and hold her; all she had was her loving mother, caring father and most of all her brothers and sisters. For some very strange reason it was hopeless that she would fall in love; Hephaestus tried once, yet failed. Athena was well known for giving advice/mentoring heroes.
POSEIDON was the Olympian god of the sea, earthquakes, floods, drought and horses. The god had numerous lovers in myth. This page describes his divine consorts including the sea-queen Amphitrite, goddess Demeter and Gorgon Medousa.
Her beauty caught the eye of the sea god Poseidon, who proceeded to rape her in the sacred temple of Athena. Furious at the desecration of her temple, Athena transformed Medusa into a monster with the deadly capacity to turn whoever looked upon her face to stone.
“Instead of punishing Poseidon, Athena turns her rage against Medusa, on the one hand because she is not able to punish the powerful Poseidon, and on the other hand because she is envious of Medusa´s beauty.”
She was affected by the evil's, becoming greedy and ambitious to rule the world after the destruction of the Greek pantheon. She could have been jealous of Zeus' authority over the world, which would also explain why she wanted Kratos to kill him.
While serving Athena, mortals and gods would both take notice of Medusa's beauty. Medusa did not take part in any of the attention she received until Poseidon, the god of the seas, took notice of her. Poseidon was enamored by Medusa's beauty, and Medusa returned the same feelings.
As the contest's winner, Athena became the city's patron deity, honored on the sacred hill of the Acropolis. Nevertheless, Athens' most sacred temple, the Erectheum, was shared between Athena and Poseidon.
Poseidon would be stronger than Athena simply because of heritage. Poseidon is one of the eldest gods, son of Kronos, the king of the titans. He is also the god of the sea, so he is in charge of a huge aspect of the world. Comparing that to Athena, Poseidon's niece through Zeus.
Despite his best efforts, Kojiro finished him off with a cross slash across his abdomen. In his final moments, Poseidon angrily cursed at Kojiro as his torso fell to pieces and his lower body fell to the ground. He eventually disintegrated and was wiped from existence, marking the god's first loss.
Poseidon then made a desperate attempt to escape and survive, but is unable to reach the edge and is grabbed by Kratos, who then proceeds to crush his eyes and finally break his neck, killing the sea god once and for all.
In all seriousness, Poseidon fears very little. He fears Zeus' authority enough to follow it, and he probably feared his father while he was alive. Apart from that, Poseidon isn't shown to fear much of anything.
By all accounts Poseidon was not specifically evil but rather just casually cruel. He frequently fought with his brother.
Answer: As goddess of wisdom and war, and protector of Athens and heroes, Athena has little to fear. In mythology, she is immensely powerful and immortal, but in The Odyssey the story of her contest with Poseidon reveals her only fear. This is the fear of being defeated.