If you didn't know any better, you'd say that Athena's distaste for a certain sea god was irrational, but the wisdom goddess is anything but. The very base of her power (and sanity) is logic and reason, so no, she is not and nor will she ever be irrational [Yes, I do mean never].
1. Not logical or reasonable; not endowed with the power of reason.
Irrationality is for children [Yes, like you.] and mortals and Aphrodite [Oh hush, no need to look so miffed.], not for prestigious goddesses such as Athena.
Still doubtful? Of course you are. You're even worse than your Uncle P. I've already found some basic, reasonable reasons for her animosity towards the upstart of sea gods.
One of the most obvious ones [We're starting with the obvious because we don't want to overload your little brain. Your mommy would be very unhappy with me if that happened.] is that Poseidon...erm, kissed Medusa in Athena's temple, therefore ruining the sanctity of it. [Medusa was once a very beautiful woman, like your mother.]
Athena was furious with him and for a good reason. You see not only is she the goddess of wisdom and patron of Athens, but she is also a maiden goddess. Meeting with Medusa at her temple...well, Poseidon's had better ideas.
But competing with her to win over Attica wasn't one of those ideas. The sea god should've known better than to challenge his niece, but then again, he was only a couple thousand years old. [For a god that's very young.]
Poseidon struck the ground with his trident and a spring came forth. It was beautiful and the people ooh'd and ahhh'd, but when they stooped to drink, they found that it was just as salty as the sea.
Athena used logic to win (of course). She gave the people of Attica an olive tree. It provided oil and food and wood, and while Poseidon's gift very pretty, it was, frankly, useless. [What? I'm not biased.] The people of Attica (later Athens) chose Athena as their patron for obvious, reasonable reasons.
Another reason why Athena doesn't like old Barnacle Beard is simply because of the Zeus-Poseidon rivalry. [That is perfectly reasonable, what are you blathering about? You know your charmspeaking doesn't work on me.] As a daughter of the sky god, she has her loyalties to uphold and can't be seen getting chummy with her daddy's greatest rival.
So yes. These are all the very clear, logical, reasonable reasons to why Athena does not like Poseidon. But still there are other reasons, just as obvious really but somehow overlooked.
Athena hates him because of his unpredictability. Oh yes, some days you can guess what he's going to say and do. But then he'll surprise you. Whether it's with a sudden spurt of maturity or coldness or rage, he'll leave you flabbergasted, even if you are a wisdom goddess and it's your job to know everything. [She does know a lot. Just not when it comes to Poseidon.]
My mother hates how constantly changing he is and how there's no pattern or cycle with his moods and personality. When she expects him to suddenly switch, he doesn't. When she expects him to do something rash, he exercises self control. Even when she expects her expectations to be wrong, she is horribly wrong. [You'll understand that sentence when you're older, Thalia.]
Athena even despises the way his eyes stay the same color but never the same shade. She hates the way he manages to look powerful in a Hawaiian shirt and Bermuda shorts and a beat up fishing hat. [That's not being shallow, and you're not one to talk.]
Poseidon isn't completely smitten with her either. He hates how she's such a know-it-all, even though its only reasonable that she is. He hates how she's always extra snobby whenever he's within a 50-foot radius of him. (You see, Athena's never right about him, so she does everything in her power to be right around him.)
It also annoys the earthshaker how she always manages to turn lovely maidens into hideous monsters that would make any warrior quake - Medusa, the gorgons, Arachne, you get the picture. Sometimes it makes him wonder if she's jealous of them but that seems absurd even to him.
All in all? They're polar opposites. She represents power of mind, strategy, craftiness, logic, control, deduction, and, well, general absence of emotion. [You didn't hear it from me.]
On the other hand, he is a loose cannon, destructive, wild power, untamable, constantly changing, predictably unpredictable (and then not), the embodiment of emotion and enough moodiness to rival a teenage girl on her period. [You'll figure out what that is when you reach puberty but that's a long way away.]
He's raw power, and she's a trained warrior. He's a bloody club, and she's a refined, sharp blade. He's a free spirit and she's a well-mannered lady. He rushes ahead and she holds back.
But if they could set aside their little grudges (like the Trojan War), they'd be powerful. In perfect balance. Just like Percy and I were, Thalia.
Poseidon and Athena could run through hell blind and walk away without a scratch. They could defeat the goddess of victory herself with both arms bound. No monster, no primordial, no titan, no giant could stand up to them. Just like Percy and I did, Thalia. [I'm not crying.]
Percy and I were more than allies and friends. We retrieved the master bolt and the helm of Hades. We found the Golden Fleece. We bore the sky for each other. We ventured into the labyrinth hand in hand. We defeated armies back to back, faced down Kronos, and lived to tell the tale. We walked through the deepest pits of Tartarus, guarding each other, and emerged with our sanity still intact (though that's not saying much). In hell, we wore monsters' horns and teeth as crowns and their hides as capes. We brought together the Greeks, Romans, and Egyptians. We distracted Gaea while our friends fought the giants. We united the mortals and brought on a new era of peace when the Mist fell. Monsters fled when they saw us and minor gods bowed when we walked on Olympus. We were the worshiped by demigod, magician, and mortal alike.
And the Olympians hated us for it. They thought we were too powerful together. Those old fools see power as danger when it's not directly in their greedy hands. So while I wasn't looking, while I was in the delivery room, they took him away from me. Snatched him right from under my nose.
They're the reason I never had the happily ever after your Uncle Percy promised me. They're the reason you see demigods and magicians point at me and whisper. They're the reason your parents and uncles and aunts think I've grown bitter and fear me for it. They're the reason Thece doesn't have a father.
And that's the final, the most reasonable reason Athena, the goddess of wisdom, and Poseidon, the god of the seas, avoid each other like the plague.
They act that way because Athena is smart, and Poseidon knows his family well enough to understand that they would throw him into Tartarus as soon as he gained more power than the king of gods. They saw the downside of having any sort of positive relationship (unlike me and Seaweed Brain).
They tried to warn us, tried to keep us apart, but we thought it was only Aphrodite playing with us again. His father once told him that any happiness he finds with any child of Athena will only result in his fall. Needless to say, we didn't listen.
So now you know better. Athena's feelings toward Poseidon aren't out of irrational hate. In fact, her condescending attitude towards him is what's keeping her uncle out of Tartarus. And his hostility is what's saving her from the wrath of his brothers and sisters.
Their act is an old one. It's a dance that's been perfected over the millenniums, and a lie so good they've almost fooled themselves. It's a brash strategy holding together by two desperate souls willing to survive. And in that sense, I think they really are allies.
[That's Jason knocking. Be a dear and tell him and your mommy that they won't be seeing me for a while and ask them to stop by every now and then to check on Thece. Oh, and let your Uncle Leo know I'd like my invisibility cap back. By tonight.
Hm? Oh, I should be back by next month, I think. Depends on how long it takes to track down the gorgons and get them to talk.
Well, I don't like talking to them, they make for horrible conversationalists - I have more fun talking to you and you're not much fun. But I'd prefer knowing that he's really down there before I jump in.
Weren't you listening? I said he was taken. Not dead, darling, there's quite a difference. I'd feel it if he was.
You see, the gods took the wrong half of a whole. They took the powerful part, thinking that it would leave me empty and suspended. But a child of Athena always has a plan, and an angered child of Athena has a plan that will not fail.
I'm going to find my husband even if I have to go through hell again, Reyna Thalia Grace. And when I do, no monster, no primordial, no titan, no giant, and no god will be able to stand against us.
1. Too powerful to be defeated or overcome.
The gods shouldn't be so irrational to think they could break up an invincible duo.]