Vyasa is considered as both, the author and a character of the Mahabharata. He plays a very critical role in the epic.
It begins with Satyavati, the fisherwoman. She was no ordinary fisherwoman. She was born when a cursed Apsara in the form of fish swallowed a kings semen. She was then raised by the fisherfolk. She was named Satyavati but teased as Matsyagandha as she smelt of fish.
Matsyagandha ferried people across river Ganga. One day she had Sage Parashar in her ferry. He expressed desire to make love to her and have children. In return he promised her a boon that she will regain virginity and will never smell of fish again. She agreed and gave birth to a child instantly with the magical powers of the sage. This child was named Krishna Dwaipayana, the dark child delivered on a river island. He was eventually known as Vyasa.
Satyavati later married king Shantanu, the father of Bhishma. From Shantanu she gave birth to two sons, Chitrangad and Vichitravirya. Chitrangad died before getting married and Vichitravirya was married to Ambalika and Ambika. However, he was unable to impregnate his wives. So Satyavati who was desperate to have her lineage as the kings of the kuru clan, calls upon her first son, Vyasa from his meditati on for years in forest. She requests him to impregnate(law of Niyoga) her daughters in law. He goes to Ambika first. Disgusted by Vyasa’s looks, she closes her eyes. Then he goes to Ambalika. She goes pale looking at him.
As a result, Ambika gives birth to Dhritarashtra who is blind and Ambalika gives birth to Pandu who is a pale weakling. Also Vyasa goes to a maid who fearlessly accepts him and the union gives birth to Vidur who is healthy and intelligent but not a royal lineage as per the law of Niyoga.