Hoel of Brittany, - of Little Britain; Howell
The Birth of Arthur, a Welsh work of the fourteenth century, makes him a son of Arthur's sister Gwyar by Ymer Llydaw (Emhyr), prince of Armorica (or Brittany). Sometimes called King, sometimes called Duke, Hoel was Arthur's cousin and ally. Lady Hoel, his wife, was killed by the Giant of Saint Michael's Mount, Brittany, who in turn was slain by Arthur, Kay, and Bedivere.
Hoel was the father both of the second wife of King Meliodas of Cornwall and of Isolt le Blanche Mains. Tristram's stepmother became his sister-in-law! Hoel's son was Sir Kehydius, who became enamoured of La Beale Isolt.
He is presumably identical with the King of Hoel of Brittany who was Tristan's father-in-law. Geoffrey calls him Arthur's nephew but L. Thorpe (Arthurian scholar) argues that this is a mistake and we must read 'cousin' for 'nephew', his mother really being the sister of Ambrosius rather than Arthur. Traditional Breton dating places his rule between AD 510 and 545. The Prose Tristan makes him the father of Runalen.
Hoel landed at Southampton with a large army in response to Arthur's request for help. He became ill during the campaign and was left at Alclud (Dumbarton) where he was besieged by the Scots and Picts. Arthur came to relieve him. Together they defeated the Saxons at Lincoln, at Caledon Wood and at Badon. They put down the Scots, Picts and Irish in Moray and toured Loch Lomond. Next they raised the siege of York, where Colgrin, the defated Saxon leader, had taken refuge, and, having defeated him, Arthur restored that city to its former glory, and returned their lands to the tree disposessed Yorkish princes, Loth, Urian and Auguselus.
Hoel also helped Arthur to defeat the whole of Gaul within nine years, and to establish his court and a proper legislative government in the city of Paris. He was a great supporter of Arthur in his wars against Rome.
Cousin or nephew
King Meliodas' second wife (Tristram's stepmother); Isoud la Blanche Mains
Hoel | The Legend of King Arthur