Character Analysis
King Alla, the Northumbrian king who marries Custance, is one sharp cookie. He instantly suspects a "greet motif," (hidden motive), when the treacherous knight accuses Custance of murdering Donegild.
And that's just as it should be. It's a king's job to be wise and fair in matters of justice. In fact, Alla fulfills all his roles exactly as he should.
A True Christian King
After Alla's conversion to Christianity, he always displays true Christian ideals. He handles the (um, false) news of the birth of a demon-child to his wife with Christian patience. He says that he welcomes whatever God sends him and trusts He'll provide a more suitable heir later on. Which, impressive. And when Alla discovers his mother's treachery he is wracked with guilt for having her executed, and does just what a good Christian ought to—he makes a pilgrimage to Rome and places himself at the Pope's command. In those days, that was what a good king did.
Finally, when confronted with the possibility of Custance's survival, Alla's inner torment is quite moving: "I oghte deme, of skilful juggement, / That in the salte see my wif is deed" (1039-1040). The guy just can't quite believe that his most heartfelt prayers might have been answered. In the end, of course, the strength of Alla's Christian faith wins out, and he acknowledges that the God who saved his wife on the open seas once before would be able to do it again. Which He does.
A Flaw, Perhaps?
The only thing we can really blame Alla for is not keeping a better eye on his mother. But, hey, it's his mother, what're you gonna do? Alla is such a stand up guy in all other respects that we'll let that one slide.