Fae Kiths

At the dawn of the First Age, the Fair Folk were all essentially unique beings. A Fair Folk is usually going to have traits in common with his/her parents and siblings, but they had powers, aptitudes, weaknesses, and cosmetic differences that stand apart.
As the Fair Folk got more factionalized, more Fae chose to associated with Fae like themselves and especially to mate with Fae like themselves. It is joked that Fair Folk are so narcistics many are only sexually attracted to beings like themselves.
Eventually selective breeding creates groups of Fae that look and act very simlar with similar power and weaknesses. When a group naturally forms like this, the grouping is called a kith. Like most things concerning the Fae, kith defy easy classification. Kith are partially a species, partially an extended family, and partially a political faction. Kith are all these things and none of the things.
Now in the Feudal Era of the Third Age, a slim majority of living Fair Folk are part of a Kith.

Echoes

All Fair Folk have Echoes which are unusual weaknesses. Fair Folk tend to develop more Echoes as they grow older, especially if they interact with the material plane a lot. Unique fae tend to develop unique Echoes. This causes unique problems but in most cases it's not especially difficult for a kith-less Fair Folk to keep his/her Echoes secret.
Fair Folk with defined Kiths are born with an Echo, and every member of the Kith has that Echo. That means once one individual Echo is exposed, the entire Kith's weakness is made common knowledge, at least in part.
Fae with Kith are born with an Echo endemic to their Kith, but they can develop new Echoes personal to them. One thing that is noteworthy is that Fair Folk with defined Kiths are somewhat innoculated against gaining new Echoes and gain new Echoes far slower than "unique" Fae do.
Fae without a kith, while they have traits resembling their parents, are effectively unique in appearance.  Fae with kiths tend to be proud of their kiths and they commonly say that the kith-less Fae are innately inferior to them.  Fae without kiths tend to be proud of their uniqueness and they commonly say that they Fae with kiths are innately inferior to them.

Manifestation

Kith are similar to each other in many ways especially visually, few can look at a redcap and not say "that's a redcap."
Faeries are similar to other Fairies of the same kith but they are not clones. Similarities are obvious, but they also differ noticeably in appearance, personality, and aptitudes.

Localization

Fair Folk with a kith usually mate exclusively within their kith, but not always. Fair Folk in general have a well-deserved reputation for promiscuity. Fair Folk with kiths usually try to avoid creating offspring with other Kiths and with unique Fae. Unique Fae usually prefer to have offspring with other unique Fae, though they often gravitate towards Fae unique like themselves, potentially planting the seeds of new Kiths.
If Fair Folk of different Kiths make, they will produce hybrid offspring with traits of both, but they are likely to possess traits belonging to neither. Arguably, these hybrid Kiths classify as "unique" Fair Folk even though they probably have similarities with both parent Kith.
If a Fair Folk with a Kith produces offspring with a unique Fair Folk, the reuslt will usually be a unique Fae with some watered down traits of the Kith holding parent.
In either case, "half-breed Kith" are often shunned by those with Kiths and those without them, finding true home nowhere and often living as second class citizens in their courts and holds. Because their offspring are stigmatized, most Fae try to avoid producing these metaphysical bastards.

Type

Metaphysical, Supernatural