Nandi(one who brings happiness) the biggest devotee of Mahadev is the son of Shilada. Shilada wanted a Son like Mahadev or having qualities like him, he performed severe penanace for it. Mahadev , our Bholenaath , Aashutosh agreed.
Nandi took birth and his splendour resembled the splendour of Mahadev. One fine day, when the kid was above 6, saints come in the house of Shilada, they met Nandi and told them that the boy has a very short life and will be dying soon, when he is 8.
Shilada got tensed but Nandi did a severe penance for years and finally Mahadev fulfilled his wish. He not only removed the fear of death from Nandi but also pleased by his devotion took him as a son.
Mahadev once, pleased by the unwavering devotion and love of Nandi gave him a boon that whosoever says his/her prayers in his {Nandi’s) ears , will be able to communicate with Mahadev directly,the prayers will get directly to Shiva’s ears.
He is married to Suyasha. According to Shiv Puran, a devotee has to at first offer prayers to Nandi- Suyasha, then Kartikeya and Ganesha with their respective wives and then finally offer prayers to ShivShakti.
No one can reach Mahadev without the blessings of Nandi, the most blessed and greatest devotee of Mahadev.
Nandi is a great warrior and quite learned too.