Vidur was supposedly half brother to kings Dhritarashtra and Pandu
After death of king Vichitrveer his both wives Ambika and Ambalika are send by Satywati to her son Ved Vyas for a child .Since Vyas is scary looking the Ambika closes her eyes and Ambalika goes pale . Vyas says Ambika son will be blind while ambalika son will be pale . Vyas again asks to grant Ambika a son .She sends her maid in her place who is not frightened and gives birth to a normal son who is Vidur . Vidur was the incarnation of dharm raj. His wisdom is also called as Vidura niti.
Since he was not of royal blood he was made counsellor .Vidur played a very big role in saving pandavas life by informing them about Lakshagrah incident
Vidur was the only person who raised a voice aginst draupadis insult in the kaurav court .Krishna respected Vidur because of his devotion and proficiency in every field.However due to him not being of a royal blood he did not get his due in the Mahabharat.