There are two leader possibilities I'd suggest:
Robert the Magnificent
I know people have said Robert Guiscard or even William the Conqueror, but I like Robert, Duke of Normandy because he represents all the Normans. He was the father of William the Conqueror and has an interesting history himself with an effective rule of Normandy and was known for his piety. His life was short because he went on a pilgrimage to the Holy Land and never returned. Plus, how can you say no to that statue there
The other alternative is:
Richard the Fearless
Richard essentially centralized the Norman state, making the Duchy one of the most important areas in France. His reign was marked mostly by peace and tranquility. I know that's boring, but that's probably owed to his strength as a ruler since he had to fight other French counts (and the French King) to get that position. He also showed cunning diplomacy, focused on strengthening Normandy rather than getting involved in French politics, and built up his relationship with the church. Plus, his nickname is "The Fearless," which is badass.
Background would probably be inside the Cathedral in Rouen, although it was rebuilt so this one is a bit anachronistic:
Unique Unit:
Knights of Roland
Replaces Knight. Unit is 25% stronger in enemy territory and starts with the Blitz promotion.
Before embarking on their conquest of England, soldiers sang the Song of Roland, which celebrated the victory of a Frankish army fighting against the Muslim Saracen invasion of France. The song had symbolic meaning for them because, even in their invasion of England (which had been sanctioned by the Pope), they viewed themselves as fighting a Holy War. No doubt the Normans who fought in Muslim Sicily, the Normans who fought in the Holy Land, and the Muslims who fought against Muslim Spain took a similar inspiration from La Chanson de Roland.
OK, that's my sales pitch. Basically, I needed a name better than Norman Knight and this one fit. The Song of Roland was important for everyone, but it was undoubtedly important for the Normans. The first recorded mention of the song is in the Chronicle describing the Conquest of England. As for the stats, if there's one thing that sums up the Normans better than anything else, it's going abroad and fighting people elsewhere. They also used Cavalry a lot. If it's too powerful, cost to produce can be adjusted.
I used three pictures to give some ideas. Really, the first guy with the ax from the second guy would be different enough so it doesn't look like the regular Knight. Although, if it has a lance, the different helmet and shield will help.
Unique Improvement:
Motte and Bailey
Can build one in the territory of each city in conquered territory. Units adjacent to the improvement lose 10 HP per turn.
I tried to balance it a bit by limiting it to one per city. It's like a citadel except it doesn't require a Great General and doesn't steal territory. Otherwise, I basically borrowed directly from Firaxis's scenario.
Unique Ability:
Soldiers of the Church
+25% combat bonus when attacking cities with a different religion than the majority religion in Norman cities. Norman missionaries can choose which religion to spread.
I wanted a religion-related bonus that did not require the Normans to found a religion. Historically, the Normans adopted Christianity as a political move, but became one of the churches strongest defenders. Although the church did not always approve their actions, they often turned to them. This is particularly true in Italy where the Normans were asked to fight against the Holy Roman Empire to protect Rome (the Normans happened to sack Rome on the way, but oh well). They were also asked to conquer Muslim Sicily and to go on the Crusades. Really, they are more associated with Holy War than any other individual group.