The given name Kellen is thought to have originated from the Gaelic language, specifically from the Irish word "cailín" which means "young girl" or "maiden." This name gained popularity during the medieval period in Ireland and Scotland.

In its earliest forms, the name was spelled as "Caillen" or "Callen," and it was often used as a diminutive form of the name Caitlín (Cathleen or Kathleen). The name eventually evolved into the modern spelling of "Kellen."

While there are no known historical references to the name Kellen in ancient texts or religious scriptures, it was likely used as a common given name among the Celtic populations of Ireland and Scotland during the Middle Ages.

One of the earliest recorded examples of the name Kellen can be found in the Irish Annals, which mention a "Callen Ó Ruairc" in the 13th century. Ó Ruairc was a prominent Irish surname at the time.

Throughout history, several notable individuals have borne the name Kellen:

1. Kellen Winslow Sr. (born 1957), an American former professional football player who played in the National Football League (NFL) as a tight end for the San Diego Chargers.

2. Kellen Winslow Jr. (born 1983), an American former professional football player who played in the NFL as a tight end, following in his father's footsteps.

3. Kellen Moore (born 1989), an American football coach and former professional quarterback who currently serves as the head coach of the Chargers.

4. Kellen Clemens (born 1983), an American former professional football player who played in the NFL as a quarterback.

5. Kellen Wilson (born 1996), an American singer-songwriter and musician known for her work in the country music genre.

These examples showcase the diverse backgrounds and professions of individuals who have carried the name Kellen throughout history, from athletes and coaches to musicians and entertainers.