which letter is associated with the last person?
Monteagle's anonymous letter
Francis Tresham
As the last few details were being finalised that month, on Saturday 26 October Monteagle received an anonymous letter while at his house in Hoxton. It contained the following message:

Uncertain of its meaning, Monteagle delivered it to the English Secretary of State, Robert Cecil.

Tresham has long been suspected as the letter's author. 
who was king at the time of the gunpowder plot King James I what was this plot? It was a failed assassination attempt against King James I by a group of provincial English Catholics why did they want to kill him? So that they can install Elizabeth on the English throne as a titular Queen. who all were the accused on trial? John and Christopher Wright, Robert and Thomas Wintour, Thomas Percy, Guy Fawkes, Robert Keyes, Thomas Bates, John Grant, Ambrose Rookwood, Sir Everard Digby and Francis Tresham.
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Francis Tresham [SEP] Monteagle letter
Tresham has long been suspected as the letter's author. Mark Nicholls states that he almost certainly wrote it, pointing to the fact that once Catesby was made aware of its existence he immediately suspected Tresham and went with Thomas Wintour to confront him. The two threatened to "hang him", but "with such oaths and emphatic assertions" Tresham managed to convince the pair of his innocence, the next day urging them by letter to abandon the plot. Antonia Fraser suggests that Catesby and Wintour's decision to believe him should not be disregarded. While making his deathbed confession in the Tower of London, Tresham failed to mention the letter; an omission which in her opinion makes no sense if he is to be regarded as its author, especially considering that its recipient was by then being credited as the country's saviour.