Søren Kierkegaard was a Danish philosopher, theologian, poet, and social critic who is widely considered to be the first existentialist philosopher. He is known for his works such as Fear and Trembling, Either/Or, and The Sickness Unto Death. His writings focused on the individual's subjective relationship with the world and how it affects their emotions and actions. He is often referred to as the 'Father of Existentialism' and his influence can be seen in many modern philosophical movements.