From Melissa Marr's website:

(Keen an) —“ancient one."

Job: Summer King

Characteristics: Whether he's wearing a glamour to pass as a mortal, or if he is in his true guise of Summer King, Keenan is gloriously handsome.

As Summer King, his collar-length hair resembles strips of copper. His skin glows and shimmers like the sun. His eyes bloom like flowers, and are unnaturally green. In fact, the smells and feel of summer surround Keenan, invoking memories of lazy, sunny afternoons.

Keenan moves with a confident swagger. He knows the extent of his charm and personal beauty.

Personality: In spite of being cursed, or maybe as a result of it, Keenan is focused and maintains an outwardly upbeat attitude. He does, in fact, have a "sunny" personality; although when something occurs to make him sad or angry, he goes with that emotion.

In many ways, Keenan is a romantic. His search for a Summer Queen -- for the possessor of the other half of his magical self -- has become a search for "true love." With his Summer Queen, Keenan can keep the Winter Queen from destroying his Summer Fey, freezing the world and killing him. Doing all he can to find her is a noble goal.

Over the centuries (or possibly from the very beginning), Keenan has come to believe the Summer Queen will not only complete him magically, but in all ways. She will be a "real" Queen -- his wife, his consort, his ally and partner. For this reason, he approaches the mortal girls he chooses by courting them. He uses his beauty and charm to make them love him.

There are rules related to this search. He must offer the mortal girl a choice. He has to, ultimately, explain to her what her choices are; and he must include/inform the Winter Girl and the Winter Queen when he has made his choice. For the most part, Keenan is honest and is very careful to follow the rules of the game, as dictated by Beira.

He is extremely aware of his responsibilities toward his Summer Court and often seems as driven, if not more, to protect them, as opposed to simply besting his mother.

While he seems to have an emotional connection to all the girls he chooses; he has a sincere attachment to Donia. One of the reasons she has difficulty being around him once she becomes the Winter Girl is due to his obvious regard for her. They both believe her failure to become Summer Queen means they are not meant to be a couple. That doesn't prevent either one of them from loving the other. It doesn't prevent Keenan from telling Donia he wishes she had been The One.

History: Nine centuries agoe, his mother, Beira (the Winter Queen), killed his father (the original Summer King) before he was born. Sometime after his birth, Beira and Irial, the King of the Dark Court bound his power, dividing it in half and sending it out into the Mortal World. Keenan's task is to seek out a Mortal Girl who he thinks is likely to be his Summer Queen. Unless he finds finds her, he's unable to wake summer.

In Chapter 12, Keenan notes he'd been searching for close to a millenia. There have been five Winter Girls (mortal girls who loved him enough to risk picking up the Winter Queen's staff, but who were not the Summer Queen). There are at least ten Summer Girls (girls Keenan identified as being "the one," but who refused to pick up that staff). Instead, they became fairies and members of his court ("Summer Girls").

Donia mentions Keenan cannot come out in the winter, so he only can search for girls a few months out of the year. Also in Chapter 12, Keenan notes the search is taking a toll on him. In spite of his confident attitude, he is becoming weaker around his mother. It also seems that the emotional toll of choosing the "wrong" girl over and over is also weighing on him.

The Winter Girls love him and choose to pick up the staff. When they do, he loses them to Winter. His very touch hurts them. On the other hand, the girls that don't love him enough to risk the staff for him remain a part of his life. When they become Summer Girls, however, they lose their inhibitions and don't remain faithful.

Until Keenan finds his Summer Queen, it is clear to him he is always losing.