Note on Courts
both courts are ruled by a species called the tuatha de danann, who eventually became known as the aos si and are comparable to elves (see species for a more detailed description).

the courts are comparable to the monarchies of the mortal world but rule based on season. the seelie courts rule over the seasons of spring and summer, while the unseelie court rules over the autumn and winter.

at the head of the court is a king or queen, whose lineage are descended from nuada, leader of a previous wave of inhabitants of ireland. they came from four cities to the north of ireland—falias (spring), gorias (summer), murias (autumn) and finias (winter) - where they taught their skills in the sciences, including architecture, the arts, and magic, including necromancy (primarily unseelie).

though the monarch is the supreme ruler of their designated court, they are influenced and advised by an elected council. the members of the court who reside within the ancient citadels are pureblooded sidhe, who are becoming rarer with each generation.

during the season in which a court is in power, festivities and feasts are held within the respective kingdom. over all decisions concerning the fae are made by whichever court is currently in control.
History
the aos si (or sidhe) are the rulers of the fae court and in looks and description are very similar to (in some lore even considered) elves. they can be either seelie or unseelie.

in ancient ireland, there were more than a few races that existed. among them was the tuatha de danann. the tuatha de danann was a magical race that possessed supernatural powers. most of them were god-like creatures or divine beings that were being worshipped. this race was also known to believe in goddess danu. she was sometimes referred to as the mother.

some people claim that their origin goes back to atlantis; however, they had to leave, for the city disappearance. in ancient greece, there were texts that suspect it was meant for the tuatha de dannan. the text included the following... in ancient greece there lived a race of nomads known as the pelasgians. tribal in nature, they were seafarers who claimed to be born from the teeth of the comic snake ophion, and the great goddess danu. it reveals that the tuatha de danann came from greece. they tried to destroy the rulers of greece, the pelasgians, at that time and take over, but their attempts failed. they then had to leave for denmark before heading to ireland. the tuatha de danann then settled in four major cities; falias, gorias, finias, and murias.

the tuatha de danann brought fascinating skills and wisdom to ireland when they arrived there. they gained those skills from four wise men who resided in the four cities; one in each. senias was the wise man who resided in murias; morias in falias; urias in gorias; and arias in finias. over and beyond, the tuatha de danann brought four treasures from the four cities; treasures that were beneficial to ireland.

when they first snaked into ireland, the firbolgs were the rulers by that time. the march of the tuatha de danann surprised them, escorting to a failure in resisting them. both races fought over the rule of ireland. eventually, victory was on the side of tuatha de danann; they won the battle and took over ireland.

as soon as the milesians arrived in ireland, it wasn't long before the tuatha de danann disappeared for good. one of the theories states that the tuatha de danann did not fight the milesians at all. that was because their foretelling skills suggested that they were going to lose the country anyway. instead, they built their own kingdoms under several hills around ireland, in tir na nog, the land of the young (the otherworld). it's said that they built them long before the arrival of the milesians. this theory suggests that the tuatha de danann were what was referred to as the fairy folk of ireland. the latter was because they lived underground for good.
Appearance
generally said to be very similar to humans in height and overall appearance, usually pale, fair haired and finely dressed. aos si blood is white and is very addictive and sought after by blood drinking species and creatures of all sorts
Personality
the aos si are not bound and tied to the lands of the otherworld. they are known to travel and move their homes. they enjoy attending mortal fairs and festivals. on quarter days-- samhain, imbolc, beltane, and lughnasa, they travel in troupes along fairy roads in a procession of celebration.

their magic depends on which court they come from and are tied to the element of their alignment. spring and summer seelie sidhe have powers related to the earth or to water. autumn and winter unseelie sidhe have powers related to fire and air.

the aos si use their magic to either blend in with the humans (glamour) or to become invisible, though this tactic can be overcome by anyone with second sight or who has dealt with them before. though those who can see them are usually wise enough to be subtle with this knowledge as it is well known that the aos si value their privacy and are quick to punish those they feel are spying on them.

they are the fae most likely to kidnap humans for pleasure or for work such as musicians, midwives, bakers, and lovers. they are also the species most known for kidnapping babies and replacing them with their own young (changelings). they have been known to marry and breed with humans (though this act is more commonly unseelie and is frowned upon in the seelie court, due to the respect for blood purity). and they’re not beyond stealing whatever it is they want though this tends to be treasures, food and livestock.

their weaknesses are iron, salt and certain herbs. all aos si have one hand of power, an ability that is unique only to them. and no aos si can lie, which makes them masters of word play and loop holes. even their own contracts have loopholes, if you're smart enough to find them.
Marriage
the seelie looked at marriage and gave it a big thumbs up. it was neat and ordered and about purity and love and devotion. it was the very epitome of what it means to be seelie! except they couldn't really leave well enough alone. marriage was far too simple for the seelie. and simple things aren't ordered, there needs to be layers to cover all of the necessary nuance otherwise you may end up with a system that is not perfectly and orderly ideal for its purpose! so they expanded on the concept in headache making ways.

the main expansion they made is deciding that the word "marriage" was far too simplistic a way to speak of the many different kinds of relationship that it covers. they decided to have entirely different ceremonies, etiquette and institutions. it really was a master stroke that managed to utterly strip any pretense of romance from most of them (the seelie contend that that makes consorts truly romantic because there's no fakery).

the end result? the seelie sidhe actually have 6 marriage-like institutions. it is perfectly acceptable (and even normal) to have more than one of them (or even multiples of each one) since none of the fae, seelie or unseelie, are especially impressed by sexual monogamy. when a sidhe has multiple spouses there are complex rules of etiquette that establish an in-depth hierarchy between the family household, including special forms of address, who gets to go to what functions, proper modes of dress, serving order etc etc.

marriage - you may only have one marriage to one person. marriage is nearly always a political institution - usually houses allying or strengthening bonds between friendly houses and factions etc. the participants are not expected to love each other (though a public persona of polite affection is expected, even if it is fake) or even live together. they are expected to sleep together, but it doesn't have to be a regular occurrence (some marriages, especially distant ones, are consummated once a year on holy days).

consort - may be of any gender and of any number. a consort is a bond of affection at least (usually love). it is the resort of those whose marriage must be used for political purposes as the seelie court recognises the primacy of love and that those who cement the political structure of the court should not sacrifice love for the purpose. since this is a bond of love seelie respect it intensely, even if it is technically ranked below that of a marriage. a consort does not create any kind of political alliance.

companion - this is not necessarily sexual (but often is). usually this is a younger sidhe companioned to an older, more experienced sidhe to act like a lord/lady in waiting. unlike the previous relationships there is a clear difference in status between the 2 sidhe and the companion is expected to be submissive and/or subservient. these arrangements are rarely permanent and serve as a kind of apprenticeship for young aristocrats (in politics, life, and sometimes sex). they are also another way of forging political alliances and contacts between houses and ex-companions usually maintain affection and friendship and are considered to be political allies.

it is rare, but still possible, to have companions between 2 sidhe of equal age and status as companions - this is usually a convoluted way to form more marriage ties with different houses. it is not encouraged or desirable, but if a house is short on unattached heirs and is looking for a way to cement some alliances then a convoluted companion may be the only way since there are no limits to the number of companions a person may have while you may only have one marriage.

mistress (when in the original sidhe, the term is unisex) - there is generally some affection but primarily the mistress is a sexual partner more than anything else. most sidhe do not keep mistresses since they feel no need to give official status to their casual sexual partners. sometimes a sidhe wishes acknowledgement for his mistresses for various reasons (it can give them some of the rights and protections of his household) and the seelie's excessive order creates a method for that formal acknowledgement.)

mate - a mate is a partner for one purpose - breeding. a relatively new institution, the concept of designated mate has arisen as the birth rate of sidhe have declined. you can have consorts and mistresses and marriages but there is no guarantee of an heir. the mate relationship is very formal - both parties negotiate before the ceremony as to what potential future children will belong to what house, who will see to their upbringing and education, etc. the whole thing is very clinical and all relationship between the two will be entirely focused on producing kids. technically you can have as many mates as you want but too many is frowned upon since it can spread one bloodline too far (assuming it works).

pet/toy - non-sidhe bound to a sidhe. this relationship is officially and expressly non-sexual. but 9 times out of 10 it is entirely sexual. basically a sidhe extends some recognition to a personal companion who is not sidhe (giving them some status and access). everyone knows that the sidhe is probably having sex with the pet, but everyone pretends they're not.