Merry, the only Sidhe (pronounced "shEE") royal to be born on American soil, fearing the continuous assassination attempts on her life thinly disguised as duels, flees the Unseelie Court in a final act of self-preservation.
Her glamour (the art of magical disguise through illusion) is nearly unrivaled at court, and she is able to pass herself off as a human with fey blood.
At the beginning of the first book in the series, Merry's full name and title is Princess Meredith NicEssus, Child of Peace, Besaba's Bane.
Meredith formed her first alliance with Sholto King of the Sluagh when he is sent to Los Angeles by an unknown man (later theorized to be Cel, her cousin).
This occurs when Merry is bled by the roses that line the entrance to the throne room of the Unseelie Court.
This is struck when Merry bargains for the cure to a curse that the Demi-Fey placed on Galen, under the direction of Cel.
In A Shiver of Light, it is in fact revealed to Merry's surprise that she is in fact having triplets, one of her babes was hiding behind her siblings in the womb In Swallowing Darkness, Merry was crowned Queen of the Sluagh by Faerie, and soon was formally recognized by the Sluagh as Sholto's queen.
Merry repeatedly states, both in her own thoughts and to her guards, that she does not believe she would be accepted on either the Seelie or Unseelie thrones because of her mortality and mixed (human, brownie, and sidhe) blood, and that she would invite certain death for either herself or some of her men by attempting to rule either court.
In the most recent book, A Shiver of Light, King Sholto's death threatens Merry's place and queen of the Sluagh.
She is surprised that the Sluagh remain loyal to her and refuse to elect another king till Sholto is avenged.
Early in A Kiss of Shadows, Merry is assaulted by a human using Branwyn's Tears, a magical aphrodisiac/date-rape drug, apparently unlocking her sidhe powers.
Branwyn's Tears temporarily turns any creature into a sidhe and forces them to crave sexual intercourse.
In the process of defending herself, Merry accidentally turns Nerys into a screaming, agonized ball of inside-out flesh.
After this, Doyle gives her Queen Andais's sword, Mortal Dread, which she uses to put the inside-out night hag out of her misery.
Further, Merry re-energizes existing ancient relics of power including Queen Andais's ring and the Healing Spring.
The most amazing example of the lighter side to her magic is during one of the battle scenes in Swallowing Darkness, when Merry uses the hands of Flesh and Blood to heal wounded and dying soldiers who have fallen protecting her, showing that for every dark power there is a light use for it.