On not much evidence Rhys Gethin is said to have been Owain Glyndwr's leading general, and historians from Sir John Lloyd to Sir Rees Davies have identified him as being Rhys Gethin of Nantconwy.
Cledwyn Fychan makes an entirely convincing case that the Rhys Gethin of the contemporary historical record wasn't Rhys Gethin of Nantconwy at all, he comes up with a far more likely candidate.
The handful of reviews I've read agree that if you want to know who Rhys Gethin is, then you should read this little book. The advice seems to defeat the object of the essay, which was to make the real Rhys Gethin better known. It's not the easiest work to get hold of and you may end up, like me, getting a version with a perfect cover and binding but with contents consisting of a different essay altogether. One of the drawbacks, I guess, of getting a Welsh language book printed in Italy.
Anyway the Rhys Gethin of the historical record was Rhys Gethin of Llwyngwychwyr, Llanwrtyd, of that there can be little doubt. There are only two mentions of Rhys Gethin in connection with Owain Glyndwr in contemporary records - there was the Rhys Gethin who captured Carmarthen in the company of others, including kinsmen of Rhys of Llwyngwychwyr; and there was the Rhys Gethin who Prince Hal mentions, in a letter to his father King Henry IV, as having raised an army in Buallt with the intention of invading Herefordshire. English nationalists should note that this letter was written in French.
As early as 1401 Rhys Gethin's father, unnamed sons and various kinsmen were "deprived" of their lands in Buallt because they had risen in insurrection with Owen Glyndourdy. As for the family's military prowess, that leading military commander in the 15C French wars Sir Richard Gethin of Builth was the son of Rhys of Llwyngwychwyr.
Some points of Radnorshire interests: Was Rhys Gethin the Rees a Gytch who an English chronicler says was involved in the Battle of Pilleth? It's impossible to say. There were certainly kinship links between Rhys Gethin's family and Philip ap Rhys of St Harmon, Glyndwr's son-in-law. As so often, the contribution of East Central Wales - in this case to Glyndwr's war - is underestimated. This elegant little book goes some way to illuminating one aspect of that contribution. If only it were better known.