From the longer Wikipedia page 
Sir Gruffydd ap Rhys (c. 1478–1521) (also known as Griffith Ryce in some antiquarian English sources) was a Welsh nobleman. He was the son of Sir Rhys ap Thomas, the de facto ruler of most of south-west Wales who aided Henry Tudor in his victory on Bosworth Field in 1485 and Efa ap Henry.
In the reign of Richard III his father's loyalty was questioned to the house of York as support grew for Henry of Richmond (later Henry VII of England). Around the time of Richard's usurpation and Buckingham's rebellion in 1483, as a way of keeping Rhys ap Thomas's loyalty, Richard demanded an oath of allegiance from Gruffydd's father as well as demanding that his young son Gruffydd be in his own custody. Rhys assured Richard of his loyalty but refused to hand over young Gruffydd, who was only four or five at the time.
When Gruffydd was older he became a member of Prince Arthur's household. Henry VII of England aimed to have his son friends with influential young men with powerful fathers in Henry's kingdom, and Gruffydd's father was one of the most powerful men in Wales after the death of Jasper Tudor in 1495, and he was chosen to serve the young Prince. Gruffydd and Prince Arthur seem to have been quite close; in 1501 Gruffydd was made a Knight of the Garter, and was with Arthur when he returned to Ludlow with his new young bride Catherine of Aragon in December 1501; and was there for Arthur's death in April 1502.