Aethelred

Aethelred is Lord of Mercia, a proud young nobleman bound to Wessex through his marriage to Alfred's daughter Æthelflæd. He wants status, credit, and command of men greater than his own courage or skill deserves, and he bristles at how easily Uhtred earns the respect he craves for himself.

Toby Regbo plays him as a man perpetually outclassed by everyone around him, and lets you watch that knowledge curdle into resentment rather than humility. The show doesn't excuse his pettiness, but it makes his insecurity legible — a lord terrified of being seen as small, which only makes him smaller.