Lucas Beaumanoir may not appear in Ivanhoe until Chapter 35, but he makes a huge impression on the reader. Beaumanoir is the Grand Master of the Order of the Knights Templar. As the leader of an international community of knights during the Crusades, you know Beaumanoir has to be a rock star on the battlefield. But his wartime prowess isn't on display here; instead, we see Beaumanoir in peacetime as a stern, inflexible bureaucrat. As the leader of the Knights Templar, it's his job to keep everybody in line – and he takes his job very seriously.
In a lot of ways Beaumanoir comes across as your typical self-righteous medieval bully. You know that expression "getting medieval" on someone? Beaumanoir is that kind of guy. He has no problem with the idea of burning Rebecca alive for witchcraft; in fact, he wants to do it.
Why? Because Beaumanoir hates both women and Jews. When Beaumanoir hears that Bois-Guilbert has kidnapped Rebecca and brought her to Templestowe, it never truly occurs to him that the problem could be with Bois-Guilbert. He immediately thinks Rebecca is a temptress and a monster and decides to try her for witchcraft right away.
Beaumanoir is an anti-Semite (someone who hates Jews) and a misogynist (someone who hates women). Introducing him at this late stage in the book allows Scott to set up Rebecca's witchcraft trial, in which Ivanhoe rides to Rebecca's rescue and Bois-Guilbert suddenly dies. While Beaumanoir's judgmental nature makes a lot of these later plot points possible, his character is still more complex than we might guess.
The thing is, Beaumanoir really cares about the Knights Templar, about their honor and reputation. He wants the Knights to live up to their ideals. The organization has already grown so greedy and hypocritical that there's not much Beaumanoir can actually do to purify their ranks. He does his best to inspire his men to follow their own code of values, so when he decides to condemn Rebecca to death, he truly thinks he's doing it for the greater good of the Order. The real kicker is, Beaumanoir truly seems to believe that Bois-Guilbert is bewitched rather than obviously guilty of multiple crimes.
To Beaumanoir's credit, when Bois-Guilbert dies on the battlefield during Rebecca's trial by combat, he admits that his death must be the will of God. Beaumanoir lets Rebecca go as an innocent woman, even though he may still despise her personally. Beaumanoir's actions toward Rebecca at the end of the book prove that he is a principled man – even if he often uses those principles in a high-handed, immoral, and bullying manner.
Still, Beaumanoir gets what he deserves in the end. As punishment for the part that his knights played in Prince John's rebellion, King Richard insists that Beaumanoir take his Templars out of England. Beaumanoir and his knights march away from Templestowe solemnly, like they're okay with going, but it can't be a good feeling when your entire knightly order gets banished. That'll teach Beaumanoir to be such an unforgiving and prejudiced old creep.