On 19 December 1316, Edward II made a most unusual offer: he wrote to his cousin* Aymer de Valence, earl of Pembroke, offering to act personally as Pembroke's attorney while Pembroke travelled to Avignon to see Pope John XXII on Edward's behalf.
* Aymer de Valence (c. 1270/75 - 23 June 1324) was the son of William de Valence, earl of Pembroke (died 1296), Henry III's half-brother, and was thus Edward II's half-first cousin once removed. Aymer's mother Joan Munchensi, who died in 1307, was the granddaughter of the great William Marshal, earl of Pembroke (died 1219).
Here's my translation of the letter, with punctuation added as there's very little in the original!
Edward, by the grace of God king of England, lord of Ireland and duke of Aquitaine, to our dear and faithful cousin Sir Aymer de Valence, earl of Pembroke, greetings. Very dear cousin, we have well understood the letters and the message which you sent us by our dear and faithful Sir John Sapy and Oliver de Bordeaux, and we thank you as dearly as we can, and we are extremely grateful that you have our business so tenderly at heart. And we make known to you, very dear cousin, that we have a special interest in all the affairs which touch you, and we will be your attorney for as long as you are overseas, and we wish you to charge your people who will remain here [in England] that for the period of time while they manage the affairs which touch you that they should come to us in person, to show to us the state of your said affairs, and we will see to them in such a way that, if it please God, you will consider yourself satisfied on your return to us. And know, very dear cousin, that all the items which you have drawn up regarding our estate, we will observe them. Wherefore we beg and charge you especially that you may have our affairs tenderly at heart, which you have so well begun, and pursue them with all the diligence that you can, in the same way that we will especially do for you. And the progress of our affairs you will make known to us from time to time, with the news which comes to you, as dearly as you love us.
Given under our privy seal at Clipstone on the nineteenth day of December, in the tenth year of our reign .
How the earl of Pembroke felt about this is anyone's guess; as Professor Seymour Phillips points out, "Edward's advice and assistance on anything might be regarded as a mixed blessing." Still, as Phillips says, the letter demonstrates the remarkable level of trust and affection Edward II had for his kinsman, and it's also interesting to see that Pembroke had made some suggestions to Edward regarding the governance of the kingdom which Edward agreed to act on. Sadly for Pembroke, however, the rise of Edward's favourite Roger Damory at court in 1317 led to a considerable reduction in the earl's influence over the king, which declined even further after Hugh Despenser the Younger became all-powerful at court. In June 1322, Edward even forced Pembroke to swear an oath on the Gospels that he would always be obedient and faithful to him, because "the king was aggrieved against him for certain reasons…and could not assure himself of the earl," most probably because Pembroke had persuaded Edward to consent to the Despensers' exile in August 1321, and also made Pembroke swear that he would not ally himself against the king or "anyone whom the king will maintain," surely a reference to the Despensers. [Calendar of Close Rolls 1318-1323, pp. 563-564.] By the time Pembroke died in June 1324, on Edward II's service in France - still, despite everything, faithfully serving him - his influence over the wayward and ungrateful king was a thing of the past.
[Edward's letter is printed in French (or Anglo-Norman, rather) in J.R.S. Phillips' Aymer de Valence, Earl of Pembroke 1307-1324: Baronial Politics in the Reign of Edward II, p. 317, and see also p. 110. The original document is located in the National Archives, SC 1/49/39.]