Son of Kadru, the world-serpent on which Vishnu rests as Narayana. When she shakes her head the earth shakes. She is also called Adishesha, the primordial snake.
Shesha means leftover. From this ‚leftover‘ the new creation emerges.
Shesha incarnated as Lakshmana in the Ramayana and as Balarama in the Mahabharata, as brother of Rama and Krishna, both are Avatars of Vishnu.
Furthermore Patanjali is considered an appearance of Shesha.
Read about Shesha here