The daughter of the chief giant Yspaddaden, this maiden, whose alternate name is Niwalen, appears in the Mabinogion story of Culhwch and Olwen. Her name appears to mean 'White Track', but it may also come from olwyn - 'wheel'.
Culhwch's stepmother swore that he should love only Olwen, and his heart swelled at the very mention of her name, even though he had never seen her. So Culhwch set out to find her, first coming to a royal court, said later to have been that of King Arthur, where the gatekeeper Glewlwyd remarked that he had never, in his long career, encountered such a handsome youth.
The King sent out messengers to look for Olwen, but a year passed without any news. Culhwch then formed a party of gifted companions to help him in his search, these companions being named as Cei (Kay), Bedwyr (Bedivere), Cynddylig, Gwrhyr, Gwalchmai fab Gwyar and Menw fab Teirgwaedd, though many others seldom named, also accompanied him.
Setting out, they at length met a shepherd, whose wife turned out to be Culhwch's aunt. She knew Olwen, and, even though she had lost twenty-three of her twenty-four sons to Yspaddaden, she agreed to help Culhwch, telling him that Olwen camen to her house every Saturday to wash her hair. The due day came, and the couple met. She was incredibly beautiful, and it was said that when she walked, white trefoils sprouted behind her.
Olwen immediately agreed to his suit but told Culhwch that he must obtain the permission of her father. She warned him not to flinch from any conditions set or tasks imposed. Culhwch agreed, and the very next morning the party set out for Yspaddaden's castle. For three days the giant told them to come back the next and then, when they had turned their backs, hurled a poisoned boulder at them. They were too quick and caught the boulder, which they hurled back. After three days the giant was severly weakened by his own poison, so he agreed to hear Culhwch.
He agreed that he could marry Olwen provided he carried out a number of seemingly impossible tasks. Each of these Culhwch completed, but, as Yspaddaden continued to pile condition upon condition, Culhwch rounded up all the giant's enemies and stormed the castle. Yspaddaden was killed, and Culhwch married Olwen, the pair remaining faithful for the rest of their lives.
Olwen is also the heroine of the Mabinogion story of Culhwch and Olwen, and also of a Welsh folktale Einion and Olwen in which a shepherd, Einion, travelled to the Otherworld to marry Olwen. They had a son whom they named Taliesin.