Who is Maelwas from Here Lies Arthur and what is their importance?

Maelwas is a King of Dumnonia though the extent of his influence only is hinted at. He is described as old and weak of body. Gwyna describes him as slow to action and condemns him as almost ploddingly incompetent. Arthur's early successes nearly all come at the expense of Maelwas though the exact relationship between the two is open to interpretation because later in the novel Arthur acknowledges that he owes Maelwas tribute.