He was the one who initiated Shri Ram to start his task of killing the demons and declare the purpose of his incarnation on Earth. He was the one who was destined to unite Shri Ram and Mata Sita in Mithila. He is none other than Rishi Vishwamitr. Born as a Kshatriya, his transformation to a Brahmarshi is an enriching life lesson. It is fitting to say that Rishi Vashist was a significant reason for his transformation as one of the greatest sages. Initially it was rivalry between the two, but later they went on to be great friends. ImageSource
The rivalry between Rishi Vashist and Rishi Vishwamitr started when the latter once visited him in his hermitage and asked to grant him the divine cow Sabala. When Rishi Vashist refused, Vishwamitr attacked him, but was defeated by the spiritual power of Vashist. Vishwamitr realises that spiritual power is greater than weaponry. He renounces his kingdom and starts penance to become a Brahmarshi as Rishi Vasisth.
It is not an easy journey for him and many obstacles come. Yet, he does not loose determination. Maharaj Trishanku, the Ikshwaku king gets cursed by Vashist sons to become an untouchable as he wanted to ascend to heaven with his physical body. Humiliated, Trishanku approaches Vishwamitr to send him to heaven. Vishwamitr does the same through all his penance, but Trishanku is thrown back to earth by the devatas. Seeing this, Vishwamitr creates a new heaven in the midst of the sky named the Trishanku Swarg proving that determination makes on achieve any heights and creating a new world itself.
Vishwamitr starts his penance again. But, the next obstacle comes in the form of Menaka, the celestial dancer sent by Indr to break his penance. Vishwamitr falls for her and leads an amorous life with her. But, he realises his mistake and again gets back to penance. Indr sends Rambha to break the penance of Viswamitr, but he curses her to become a stone, thus not hesitating to learn from his mistakes. This stint of his life teaches that one should not get attracted by dubious attractions while working towards a goal.
After overcoming all such major obstacles, Brahma Ji appears to give him the title of Brahmarshi. But, Vishwamitr demands that this title is given to him by Rishi Vashist. The noble Rishi Vashist then appears in front of him and accepts him as a Brahmarshi. With this, Vishwamitr conveys subtly that he greatly regards Rishi Vashist and offers him his salutations through this gesture. Such is his intelligence when he shows his arch enemy as the one he regards the most.
With his spiritual power, he discovers that Shri Ram is the one who is worthy of all the weaponry power he gained through his penance. Although he has the power to kill the demons himself, Vishwamitr comes to Ayodhya and on the pretext of taking Shri Ram and Lakshman to kill the demons, he bestows them all his divine weaponry skills. With this, he warns the evil forces that the Almighty has arrived in the form of Shri Ram to conquer them. Shri Ram revers him greatly and serves him with utmost care and respect during their time spent with him in Siddhashram and Mithila. As Goswami Tulasidas mentions this beautifully,
तै दो बंधू प्रेम जन जीते
गुरु पद कमल पालोत्तक पीते II
Thus, blessed is Rishi Viswamitr that he is served by none other than the Almighty, who has taken the form of his dear student, Bhagwan Ram.