In Ramayana, Kaikeyi was known to be very affectionate with Rama initially. Manthara's evil council changed Kaikeyi's mind. Had it not been for Manthara, Kaikeyi would not have planned Rama's exile and Bharata's coronation. That is why this Umachi book from the Rama Series (Book 4) has been named "An Evil Plot". Manthara was responsible for sowing the evil thought in Kaikeyi's mind.
In Bhaja Govindam (a composition of Adi Shankara) There is a paragraph which mentions the following -
Sat Sangatve nissangatvam, Nissangatve nirmohatvam ....
It essentially means, Satsang or good company is the first step towards one's liberation.