Ann Moore-Martin was "gushing" about Benjamin Field, then aged 26, and acted as if she was "hypnotised" by him.
Oxford Crown Court heard Benjamin Field began a relationship with Miss Moore-Martin as part of a "gaslighting" plot a few months after allegedly murdering her neighbour, Peter Farquhar, in the picturesque village of Maids Moreton, in Buckinghamshire.
Prosecutors allege Benjamin Field's "project" was to befriend someone vulnerable, get them to change their will and then "make sure they died".
Mr Field, the church warden, 28, the son of a Baptist minister, has admitted fraud by pretending to be in a relationship with the elderly spinster but denies conspiring with his friend Martyn Smith, 32, a magician, to murder her.
Oliver Saxby QC, prosecuting, told the jury that Miss Moore-Martin confided in her sister-in-law Gillian Moore-Martin that they were a couple.
"She said she loved him. She said they would sit together on the sofa and he would put his arm round her and fluff his eyelashes down her cheek," he said.
"Ann Moore-Martin was gushing about Benjamin Field. She sounded like a lovestruck teenager. It was as if she was hypnotised by him."
Field is accused alongside Smith of plotting to make the church-going pensioner's death an accident, such as dying during sex, falling down the stairs and choking on her dentures, or suicide having got her to change her will.
"Their relationship became sexual and regularly so - indeed, this was of course one of the ways Benjamin Field envisaged Ann Moore-Martin dying; and he took a photograph of one act - this was her on him - using his phone," Mr Saxby said.
He also bought her a Sybian sex toy, the court heard.
Field would "play games" with her, acting indifferently towards her, talking of meeting a younger woman and gave her a handheld clicker so she could click it every time she thought of him.
He has admitted defrauding Miss Moore-Martin of £4,000 to buy a car but he denies a more substantial later fraud.
Later, Benjamin Field and Smith, together with his younger brother Tom Field, 24, are accused of defrauding the teacher out of £27,000 to buy a dialysis machine.
They claimed that Tom Field was seriously ill and needed the hi-tech equipment to help him study at Cambridge University, otherwise he would die.
As part of the dialysis fraud, Benjamin Field is accused of writing messages with white marker pens on mirrors in the deeply religious Miss Moore-Martin's home purporting to be from God.
"Ann Moore-Martin said she thought they were messages from God," Mr Saxby said.
"In her mind, it was a validation for giving money to Tom Field for the dialysis. Sometimes, the mirrors moved."
Benjamin Field and Smith are accused of "psychologically manipulating" the pensioners, making them believe they were losing their minds and left so desperate would kill themselves.
Benjamin Field had also referenced a book by 17th-century Spanish author Baltasar Gracian called How to use your enemies.
Mr Farquhar, a former English teacher, author and part-time lecturer, died in October 2015 while retired teacher Miss Moore-Martin passed away in May 2017.
Mr Saxby alleged that Benjamin Field suffocated Mr Farquhar and tried to kill Miss Moore-Martin "by a manner of means".
He had even written a suicide note purporting to be from her but she later died from natural causes.
"There is no doubt he wanted her dead. He was promiscuous in the idea of death," Mr Saxby added.
Benjamin Field and Smith deny charges of murder, conspiracy to murder and possession of an article for the use in fraud.
Field, of Wellingborough Road, Olney, Buckinghamshire, also denies an alternative charge of attempted murder. But he has admitted four charges of fraud and two of burglary.
In addition, Smith, of Penhalvean, Redruth, Cornwall denies two charges of fraud and one of burglary.
Tom Field, also of Wellingborough Road, Olney, Buckinghamshire denies a single charge of fraud.
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