Kellen became King of the Elves after the death of his father Kylen Elessedil in the war between the Federation and the Free-born. Kellen hated the Ard Rhys Grianne Ohmsford even more than Kylen did, because he blamed her for his father's death.
Like his father, Kellen also greatly disapproved of his uncle Ahren Elessedil for having become a Druid and an ally of Grianne, and because Ahren had once tricked Kylen into agreeing to send two dozen Elves to Paranor to help revive the Druid Order. Kellen also disapproved of his younger sister Khyber Elessedil for spending time with Ahren.
As King, Kellen was easily tricked by the Federation into launching an attack on their forces. He even took his two sons, Kiris and Wencling, with him despite the attempts of Pied Sanderling, the Captain of the Home Guard and Kellen's cousin, to dissuade him. Kellen's airship the Ellenroh was incinerated in a fireball as the Federation tested their new weapon, resulting in his death and the death of his sons and the entire crew. With his death, his widow Arling Elessedil became Queen.
Resentful of the Druids for their support of the Federation and Prime Minister Sen Dunsidan, the parties responsible for the death of her husband and sons, Queen Arling cuts off all ties between the Elves and Druids, even though this support was only given to the Federation during the Druids' brief time under Shadea a'Ru's leadership. Over a century later, Elves like Aphenglow Elessedil who are are Druids themselves are seen as dangerous outcasts.
In Straken it is mentioned that Queen Arling would likely be succeeded by her and Kellen's daughter, as all male heirs were killed. However, in Wards of Faerie, a retroactive continuity was established where Arling was said to have been succeeded by a son named Emperowen. Emperowen also has a brother named Ellich Elessedil, which means that Arling had two more Elessedil sons besides Kiris and Wencling. Presumably this retroactive continuity means that Emperowen and Ellich were sons of Kellen as well as Arling, as they would not have had Elessedil blood otherwise.