Changeling

A changeling is a person with one human parent and one faerie parent.

Description
In childhood, changelings are indistinguishable from ordinary humans. In adolescence, however, a changeling's faerie ancestry begins to express itself, and the changeling starts to exhibit faerie characteristics in line with his or her fae parent's nature. For example, Meryl, a troll changeling, began to display troll characteristics such as prodigious strength, uncontrollable anger, and troll-like hair coloration.

At any time, a changeling can choose to fully embrace one side of their nature, becoming either fully human or fully faerie. Until the changeling makes such an election, he or she is are under the rule of the Faerie Court to which his or her fae parent belongs.

Summer Knight
In Summer Knight, Harry Dresden finds a photo of four changeling friends with the murdered Summer Knight, Ronald Reuel at Disneyland, putting Harry on their trail.

At the funeral parlor where Ronald Reuel is laid out, Harry Listened—they heard Dresden's fair, but they're scared of him. When Harry finds them in the alley, they toss him into a trash bin and run. Later, Meryl and Fix (sans Ace) apologize for earlier and hire Harry to find Lily. Meryl and Fix go with Harry and the Alphas to save Lily in the Valley of the Stone Table.

Cold Days
In Cold Days, Harry gets attended to and nursed back to health by Sarissa. Mab told Harry that she is a changeling who once sought of her a favor.

External references

 * Changeling - Wikipedia
 * Tales of Faerie: Changelings
 * Irish Fairies | Changelings
 * Irish Faerie Folk of Yore and Yesterday: The Changeling | Got Ireland
 * Irish Changelings – Folklore in Ireland – Changelings Folklore