Edward de Courtenay, 3rd/11th Earl of Devon
Edward de Courtenay, 3rd/11th Earl of Devon (c.1357 – 5 December 1419), known by the epithet the "Blind Earl", was the son of Sir Edward de Courtenay and Emeline Dawnay, and in 1377 succeeded his grandfather, Hugh Courtenay, 10th Earl of Devon, as Earl of Devon.
The ordinal number given to the early Courtenay Earls of Devon depends on whether the earldom is deemed a new creation by the letters patent granted 22 February 1334/5 or whether it is deemed a restitution of the old dignity of the de Redvers family.
Authorities differ in their opinions, and thus alternative ordinal numbers exist, given here.
Hugh de Courtenay, 4th/12th Earl of Devon
Hugh de Courtenay, 4th/12th Earl of Devon (1389 – 16 June 1422) was an English nobleman, son of the 3rd/11th Earl of Devon, and father of the 5th/13th Earl.
The ordinal number given to the early Courtenay Earls of Devon depends on whether the earldom is deemed a new creation by the letters patent granted 22 February 1334/5 or whether it is deemed a restitution of the old dignity of the de Redvers family.
Authorities differ in their opinions, and thus alternative ordinal numbers exist, given here.
Thomas de Vere, 8th Earl of Oxford
Thomas de Vere, 8th Earl of Oxford( c. 1336 – September 1371) was the second son of John de Vere, 7th Earl of Oxford, and Maud de Badlesmere.
He was predeceased by his elder brother, Sir John Vere of Whitchurch, Buckinghamshire, who married Elizabeth de Courtenay, the daughter of Hugh de Courtenay, 10th Earl of Devon, and died before 23 June 1350 without issue.
Thomas took part in several of the military campaigns of Edward III.
He married, Sometime before 10 June 1350, Maud de Ufford, daughter and heir of Sir Ralph de Ufford and Maud of Lancaster, the daughter of Henry of Lancaster, grandson of King Henry III.
After Thomas ’s death, his widow was indicted for involvement in a plot against King Henry IV, but was later pardoned.
When Thomas died in 1371, he was succeeded by his only son, Robert de Vere, 9th Earl of Oxford.
Hugh de Courtenay, 1st/9th Earl of Devon
Hugh de Courtenay, 1st/9th Earl of Devon (14 September 1276 – 23 December 1340) was the son of Sir Hugh de Courtenay (died 1292), feudal baron of Okehampton in Devon, by his wife Eleanor le Despenser (died 1328), sister of Hugh le Despenser, 1st Earl of Winchester.
Forty-one years after the death of Isabel de Redvers, "suo jure" 8th Countess of Devon (died 1293)
(eldest daughter of Baldwin de Redvers, 6th Earl of Devon), letters patent were granted by King Edward III of England, dated 22 February 1335, declaring him Earl of Devon, and stating that he 'should assume such title and style as his ancestors, Earls of Devon, had wont to do so'.
This thus made him 1st Earl of Devon, if the letters patent are deemed to have created a new peerage, otherwise 9th Earl of Devon, if it is deemed a restitution of the old dignity of the de Redvers family, and he is deemed to have succeeded the "suo jure" 8th Countess of Devon, his second-cousin once removed.
Authorities differ in their opinions, and thus alternative ordinal numbers exist for this Courtenay earldom.
Richard de Redvers, 2nd Earl of Devon
Richard de Redvers, 2nd Earl of Devon( died 1162) was Earl of Devon from 1155 until his death and was feudal baron of Plympton in Devon.
He married Denise, one of the daughters and coheiresses of Reginald, Earl of Cornwall.
He was High Sheriff of Devon from 1154 – 1157.
He was the son of Baldwin de Redvers, 1st Earl of Devon and brother of William de Reviers, 5th Earl of Devon.
He was succeeded by his son Baldwin de Redvers, 3rd Earl of Devon( died 1188).
Robert de Vere, 5th Earl of Oxford
Robert de Vere, 5th Earl of Oxford( c. 1220 – 1296) was the son and heir of Hugh de Vere, 4th Earl of Oxford, and chamberlain to Queen Eleanor.
William Courtenay, 9th Earl of Devon
William" Kitty" Courtenay, 9th Earl of Devon( c. 1768 – 26 May 1835), was the only son of William Courtenay," de jure" 8th Earl of Devon, 2nd Viscount Courtenay and his wife Frances Clack.
He attracted infamy for a homosexual affair with art collector William Beckford from boyhood when it was discovered and publicised by his uncle.
From October 1788 until 1831, his official title was The Rt. Hon.
The Viscount Courtenay of Powderham.
Hugh de Courtenay
Sir Hugh de Courtenay (1251–1292) was the son and heir of John de Courtenay, feudal baron of Okehampton, Devon, by Isabel de Vere, daughter of Hugh de Vere, 4th Earl of Oxford.
His son inherited the earldom of Devon.
Hugh de Vere, 4th Earl of Oxford
Hugh de Vere, 4th Earl of Oxford( c. 1207 – December 1263) was the only son and heir of Robert de Vere, 3rd Earl of Oxford and Isabel de Bolebec, daughter and eventual sole heiress of Hugh de Bolebec.
Hugh Courtenay (died 1425)
Sir Hugh I Courtenay( after 1358 – 5 or 6 March 1425), of Boconnoc in Cornwall and of Haccombe in Devon, was Sheriff of Devon for 1418/19 and was thrice elected knight of the shire for Devon in 1395, 1397 and 1421.
He was a grandson of Hugh de Courtenay, 2nd/ 10th Earl of Devon( 1303 – 1377), was the younger brother of Edward de Courtenay, 3rd/ 11th Earl of Devon( 1357 – 1419)," The Blind Earl", and was the grandfather of Edward Courtenay, 1st Earl of Devon( d.1509), KG, created Earl of Devon in 1485 by King Henry VII.
He was the link between the senior line of the Courtenay Earls of Devon made extinct following the Battle of Tewkesbury in 1471( his elder brother's line) and the post- Wars of the Roses creation of a new Earldom for his grandson made in 1485 by King Henry VII.

Question: Who is Hugh De Courtenay, 1St/9Th Earl Of Devon's paternal grandfather?
Answer: John de Courtenay