Lopamudra was a female Rishi who revealed Vedic mantras and did lots of Tapas, as per Mahabharata:
And obtaining her as wife, Agastya addressed Lopamudra, saying, 'Cast thou away these costly robes and ornaments.' And at these words of her lord, that large-eyed damsel of thighs tapering as the stem of the plantain tree cast away her handsome and costly robes of fine texture. And casting them away she dressed herself in rags and barks and deerskins, and became her husband's equal in vows and acts. And proceeding then to Gangadwara that illustrious and best of Rishis began to practise the severest penances along with his helpful wife. And Lopamudra herself, well pleased, began to serve her lord from the deep respect that she bore him. And the exalted Agastya also began to manifest great love for his wife.
After a considerable time, O king, the illustrious Rishi one day beheld Lopamudra, blazing in ascetic splendour come up after the bath in her season. And pleased with the girl, for her services, her purity, and self control, as also with her grace and beauty, he summoned her for marital intercourse.
Did Lopamudra ever receive Upanayana to do the tapas?