Srimad Bhagawatham does not mention Radha by name at all. It does speak of a Gopi who speaks the Bramara Gita when Uddhava comes visiting and has been held to be Radha.
Radha, on the other hand, has an immortal place in the legends of Krishna, and as SP has so beautifully pointed out, there is Seetha Rama, and Lakshmi Narasimha but not Rukmini Krishna, only Radha Krishna, the Radha that He never married.
Radha is said to be Dhara, the Earth, and hence there is a tale that links her to Satyabhama who is also a manifestation of the Dharthi.
Some texts also hold Srimati Radharani above even Rukmini and Satyabhama etc and hold that it these 16108 'wives'of Krishna were all manifestations of Srimati Radharani
Some people hold Radha to be a paternal aunt of Krishna, even and that she certainly was elder to Krishna by several years. That when the lad Krishna was 8 (during the time of the Rasa), she was nearly 30 or 35.
The stories also have it that Radha was so imbued in Krishna tatwa that she herself became one with Krishna. That her fair complexion was lost and she became dark like Him.
But one thing is held to be the Truth amongst all these versions. That Radha represents Prakriti and Krishna, the Purusha. Radha, the Jeevatma and Krishna, the Paramatma.
The Puranas have several stories - amongst these, some stories are just stories, others are not so much stories but symbols. In those kind of stories, it is better to not give more importance to the story as much as to the truth behind the story. Indeed, some of these stories probably even need to censored! But such stories too have found place in the Puranas only to cater to everyone, the masses.
Just as even a serious film suddenly breaks into a hip swiveling, pelvic thrusting dance sequence, the Puranas suddenly go off into a spicy titillating story.
But behind these stories too there is a deeper truth. The authors of these texts were clear in the vision - we present the story here, with the truth behind it. Those who can get beyond the story to the truth alone deserve the truth, the others can be entertained by the mere story.
To know the hidden truth, there is no recourse but the Guru, as Krishna Himself says.
"Tat viddhi prani patena pari prashnena sevaya
Upadekshayanti te jnanam jnaninaastatwadarshinaha"
Learn this truth by humble prostrations, questioning and service to the jnanis, the tatwadarshis who will impart it to you.
Thus, separating the chaff from the grain with the help of the Guru, the understanding dawns upon one that Krishna Leela is happening in the hrudaya of everyone all the time. The Jiva is Radha, the Antaryami is Krishna. When the Jiva longs for Him, He shows Himself.
Then, suddenly, He hides Himself after showing a glimpse. The Jiva then yearns for Him, pines for Him and secures a oneness with him in the end.
With Salokya first, then Sameepya, then Sarupya and finally, Sayujya, Radha represents the journey of the soul to its source.