Of all the Olympians, it can be said that Poseidon's the proudest, and also the most thwarted. The son of Cronus and Rhea, and one of Zeus's elder brothers, he vied for rulership of the pantheon but failed. Poseidon later tried to win the patronage of several mortal cities, and usually failed in that as well, as the immortals judged other powers more suited to the task. For a power who so desires the praise of mortals, each of these rejections came as a serious blow to his pride. Perhaps that's why Poseidon has become so vindictive, and why he flies into rages so easily.
Fact is, his temper's notorious among the Olympians. Like the sea, he can change from placid to furious in a matter of seconds, and woe to the berk who tries to hold him back. Poseidon's passions are the sea, its strange currents, and all the creatures within it Some say he willingly took the rulership of the sea because it, at least, accepted him - the creatures of the ocean saw him as one of their own.
The Olympian has sired numerous children, though most of them were disappointments, turning to the ways of evil and the cross-trade. Still, Poseidon is their father, and he does what he can to avenge the wrongs done to them. Sadly for the sea king, Zeus (or one of the other Greek powers) protects those who slay Poseidon's children, as long as the killer is a hero or upholds the general law of the land. It's just another instance of Poseidon being thwarted by his fellow powers.
The blood has precious few friends among the gods at all. He's got a tenuous truce with Deep Sashelas of the elves, but it's wearing a bit thin as Poseidon tries to poach the worship of the sea elves. He's friendlier with Manannan mac Lir of the Celts than he's been in eons, but chant is a feud simmers just below the surface of their relationship. Both gods are possessive of their domains, and neither one's willing to give an inch.