Agastya

Maharshi Agastya was son of Pulastya and elder brother of Sage Vasishta. He married daugher of Vidarbha King called Lopamudra also called as Kausitaki and Varaprada. Therayar and Tholkappiar were two of his diciples.
Many mantras of Rig Veda, most ancient text available, were compased by Sage Agastya and his diciples.
One of the earliest treatise on Tamil grammer called Agattiyam or Agastyam was composed by Sage Agastya. Lord Siva himself taught Tamil language to Agastya.
He had many yogic powers and siddhis.. He once turned King Nahusha into a serpent and restored him to his original form afterwards.
He used to stay in a hermitage on mount Kunjara situated south of Vindhya mountains.
He was known to have eaten up a Rakshasa named Vatapi who wanted to kill the Sage with his special powers.
Lord Rama along with Sita devi and Lakshman visited the sage in his hermitage. Agastya accompained Lord Rama to Ayodya on his return.
The Lalitha sahasranama(1000 names of devi,used to chant), which describes the 1000 names of the mother Goddess (Known commonly as Gowri, Parvati or Durga), was first revealed to the world when Hayagriva, a manifestation of Vishnu, taught the same to Agastya.
Agastya is said to have composed the Aditya Hridayam, a hymn to Surya, and taught the same to Rama just before the war between Rama and Ravana.

Once he asked Vindhya mountains, which were growing in height day by day, to facilitate his passage from North to South India. Vindhya mountains obliged the sage and bent low enough to enable him to pass over and promised not rise until Sage returns. Agastya settelled down in the south permanently and true to it's word, Vindhyas nerver grew further.