My question concerns radha, the companion of Krishna. I have heard that Krishna and radha were companions since childhood. But I want to know what happened to her when she grew up. Thank you
Krishna and Radha are eternal consorts as per texts. They live eternally in their abode called Goloka Vrindavan and it is said the goal of a person's life is to be part of that planet.
Krishna and Radha represent 100% pure unconditional love and are famously referred to as the "all attractive couple". Radha Rani is spoken very less in all scriptures and texts because she is the most precious person. Krishna chases her and she represents bhakti/devotion, so in other words it is implied that Krishna chases devotion. So she is a well kept secret of Krishna.
When they descended to Earth they were born in the same village and played pastimes and loved each other 100%. The whole leela of Krishna going away to defeat Kamsa, is to show that aspect of love during separation, in this case, a very long separation. Radha Rani understands this and decides to experience the sweet nectar of longing for her love during separation. This is another aspect of the 100% love situation - something which will never happen in the material world. Radha Rani loves him despite whatever happens with him and Krishna loves her back equally. They both enjoy aching for each other under separation. They cry, they feel lost, they undergo all these arduous feelings just to test their love time and again.
Meanwhile Radharani marries a dumb Yadava called Abhimanyu and has quite some bad encounters with her in-laws. But being Radharani she takes it in her stride very well.
Jatila-devi is Radharani's mother-in-law and Abhimanyu is Radharani's so-called husband. Kutila-devi, who is always eager to find fault, is Radharani's sister-in-law.
Abhimanyu does not realize the value of the diamond he has with him. Krishna anyway does not care, neither does Radha. They both only have eyes for each other. There is a pastime where Radharani is in her room and Krishna is being transported inside a basket full of flowers by her Gopi friends into her room and they spend the night in peace - this is after Radharani's marriage.
There is no problem for Radharani to survive because she represents 100% Hladini energy aka pleasure potency of Krishna. So she can be ever smiling and ever caring always in his remembrance. Krishna when he marries Rukmini, Satyabhama and other wives, he does it in the Visnu duty form. But the highest of all is Radharani in his heart. Others are all by association with Visnu duty form. There is also another story where Rukmini repeatedly questions Krishna as to why he keeps exclaiming Radhe Radhe in his dreams.
They also say the only way to get into Krishna's heart is to please Radha. That is why people say "Radhe Krishna" and also say that a Krishna idol without Radha is incomplete. In the Srimad Bhagavatam, Sukhdev Goswami who is Vyasa's son is actually the eternal pet parrot of Radharani who is watching the love between Krishna and Radha non stop. That love is 100% pure love and hence Sukhdev Goswami only mentions Radha in the Srimad Bhagavatam, but without taking her name. If he takes her name, he gets into the trance of transcendental ecstasy which he won't be able to come out of. She is referred to as the principal gopi.
anayaradhita nunam bhagavan hariir isvarah
yan no vihaya govindah prito yam anayad rahah
They also say there is something much higher and more sweeter than the politics of Mahabharatha and that is Krishna's life with Radha in Vrndavan.
There are actually many speculations as to what happened to her, since she disappears quiet abruptly after lord Krishna's childhood saga, where she played a very pivotal role.
So she remains a mystery yet till now with nothing definite to settle for, so I will be giving you 2 accounts of what have I come across about her :
As described in Was Radha aunt of Krishna? post, Radha was elder to Krishna and her shadow was already married to Yashoda's brother Raayana and real Radha was companion of Krishna.
46 In the sacred forest of Vrndavana in the land of Bharata, Krsna the lord of the cowherds enjoyed the company of Radha in Vrndavana for a short duration.
47 Thereafter because of the curse of Sudama both of them were separated from each other.
48 After the lapse of a hundred years, both Krsna and Radha met each other while on a pilgrimage.
49 Thereafter lord Krsna who was well-versed the knowledge of tattva went back to Goloka together with Radhika and her mother named Kalavatl, besides all the cowherdesses and Yasodha the mother of Krsna.
50 Vrsabhanu, Nanda, all the cowherds and cowherdesses who had arrived on earth from the Goloka went back to their abode.