Manthara in Ramayana was the old maid servant of Queen Kaikeyi. It was her cunning and deceitful nature that poisoned the mind of Kaikeyi and which eventually resulted in the exile of Bhagvan Sri Ram. The story of the previous life of Manthara and why she was deceitful and cunning is mentioned in the Ramopakhyan in the Mahabharata.
To console Manthara, Bhagvan Sri Ram narrated her story. In her past life she was a beautiful Gandharvi – celestial beings who entertained the Devas through music and songs. She was told by Brahma to take birth as Manthara and was the given the mission to exile Sri Ram to forest so that he can bring end to the rule of the demons and convert them to the path of Dharma.
Bhagvan Sri Ram also told her that in her next life she will be known as Trivakra and this will happen when Krishna Avatar will take place on earth. Sri Krishna will then embrace her and this will straighten her back and she will become beautiful again.