Keats’s first mention of Fanny is in his journal letter to his brother George on December 16, 1818. Keats thought her ‘beautiful and elegant, graceful, silly, fashionable and strange we have a little tiff now and then — and she behaves a little better, or I must have sheered off.’

It had not yet dawned on him that he lacked the emotional stamina to sheer off, as Fanny’s qualities were exactly those he needed. Keats had long sought a woman who understood his temperament, and loved him for his own sake. Isabella Jones aroused his interest, yet he suspected she looked upon him as a stepping-stone in the literary arena, and her enigmatic behaviour was intimidating, whereas Fanny’s was stimulating, and epitomised disdain. Keats had been susceptible to fashion and pretty women, but he was not familiar with apathy or overt disaffection. Fanny’s allure stemmed from her originality and her youthful couldn't-care-less attitude.

Keats remembered his meeting with Fanny as love at first sight. He would have told her so but thought she disliked him. In a letter to Fanny on July 25, 1819, eight months after their first meeting, Keats wrote:

When Keats met Fanny she had just progressed from adolescence to independence and sophistication. Keats, on the other hand, was an anguished young man whose search for mental and monetary security seeped through the spontaneity and uneasy rapture of his love letters and placed them among the most touching of their genre in English literature. Their underlying longing and forlorn restlessness elucidate the process of his yielding to love and the evolution of his poetic skill. Fanny, twenty years later, gave her own first impression of Keats: ‘His conversation was in the highest degree interesting, and his spirits good, excepting at moments when anxiety regarding his brother’s health dejected them.’

Interestingly, the emotional attachment between the two was not viewed in a positive light by their family and friends. Fanny's widowed mother was anxious that her two daughters should marry well. She did not view Keats, who in the months that followed their meeting seemed to occupy Fanny’s thoughts more and more, as an ideal candidate. His first collection of poems, published in 1817, had flopped. Fanny’s aunt thought the poetry a ‘mad craze’. In addition, Keats’s literary friends disapproved of Fanny: John Reynolds was later to write to Keats’s publisher, John Taylor, about the poet’s departure for Italy:

It was during the year 1819 that all Keats’s greatest poetry was written — Lamia, The Eve of St. Agnes, the great odes (On Indolence, On a Grecian Urn, To Psyche, To a Nightingale, On Melancholy, and To Autumn) and the two versions of Hyperion. This poetry was composed under the strain of illness and his growing love for Fanny and is often mirrored in various words and phrases he uses in his letters to her. The Eve of St. Agnes was written in the first rush of his meeting with Fanny Brawne, and it conveys an atmosphere of passion and excitement in the description of the elopement of a pair of young lovers.

In spite of their deep feelings for each other, Keats had no financial prospects that would let him marry. He resolved to concentrate on writing to make money and a name; but to do this, he decided he must move away in order to live cheaply and without distraction. In June he travelled to the Isle of Wight, off the south coast of England, seeking sufficient peace and quiet to write a play. He wrote of his worries in letters to Fanny. His ‘unguided fate ... spread as a veil’ between them. Returning briefly to London to sort out a family problem, he did not allow himself to travel the few extra miles to see Fanny explaining in a letter to her, ‘I love you too much to venture to Hampstead. I feel it is not paying a visit, but venturing into a fire.’ But on October 10 he saw her again and was overwhelmed:

Keats’s sonnets ‘Bright Star’ and ‘The day is gone and all its sweets are gone’ were probably the result of his renewed awareness of the fascination Fanny Brawne held for him. The sensuousness and keen passionate ardour of the words are emblematic of the agonised intensity of Keats’s love.

On October 13, 1819, Keats wrote:

To Fanny, also composed about this time, expresses the confused intensity of Keats’s feelings and, like the letter quoted above, manages to suggest a rueful element in his emotional surrender.

Keats’s fear that his poetic ambition and passion for Fanny are at variance is explored again in ‘I cry your mercy’ (c. Letter of 15-31 Oct. 1819):

The winter of 1819-20 was miserable. On February 3, 1820, Keats came home from London, desperately ill. He went to bed, coughed, and saw blood on the sheet. Charles Brown brought a candle. Together they looked at the blood. Keats said, ‘I know the colour of that blood; it is arterial blood ... That drop of blood is my death warrant.’ That night he suffered a second huge haemorrhage. Absolute rest was necessary to postpone death. Excitement could kill him, so Fanny communicated by notes and small gifts, a vision beyond the window, a visitor who could not stay. Poignantly he wrote, ‘I shall follow you with my eyes over the Heath.’

As the disease took deeper hold, his moods became more extreme. He worried about Fanny. ‘Do not I see a heart naturally furnished with wings imprison itself with me?’ When she was away he felt jealous, but his jealousies were ‘agonies of love’.

Cutting short a brief, chaotic stay with his friend, the poet and essayist Leigh Hunt, in nearby Mortimer Terrace, he walked back to Hampstead, arriving exhausted and in tears. Mrs. Brawne took him in, and for his last month in England, she and Fanny nursed him in their home and helped him prepare to go to Italy, where warm skies and dry air might save him. With death on the horizon, the love between Keats and Fanny was at its most powerful, enough to ‘occupy the wildest heart’ as Keats had written.

During the voyage to Italy, he wrote to Brown,

Later, from Naples he wrote:

It was from here that he wrote to Mrs. Brawne assuring her of his appreciation of her kindness and sending her blessings to Fanny. This was the last time he had the courage or morale to write to Fanny.

As Keats lay dying in Rome, throughout his last days, he held a large white carnelian, a precious stone, that Fanny had given him to cool his fever. He refused to open the letters that Fanny continued to send, as he feared that they might bring on a fatal haemorrhage or intensify his black despair. On February 23, 1821, Keats died, in the arms of his painter-friend, Joseph Severn. All the mementoes of Fanny, including the unopened letters, were buried in Keats’s grave, under the winding sheet, on his heart, just as he had wished.

Fanny Brawne, later Fanny Lindon, mourned for Keats till the day of her death.