Well, this novel is called Ivanhoe, so it's pretty easy to make a case that Ivanhoe is the main character. It's his return to England, his fights with Bois-Guilbert, and his love triangle with Rebecca and Rowena that give the book its plot. We would say that Ivanhoe is a pretty central figure here.
While Ivanhoe is an excellent candidate for protagonist, he is not our only option. Rebecca has her own, mostly independent plot line. As an educated woman who has to balance the greed of her father with the prejudices of the whole world against her people, Rebecca is one of the most complex and interesting characters in Ivanhoe. Her struggle to maintain her Jewish faith against the bullying of Bois-Guilbert makes her one of the only truly religious people in the novel, much more so than the many monks and friars drunkenly running through these pages. We don't want to sound biased, but Rebecca is truly an amazing and well-rounded character who deserves the spotlight whenever we can give it to her.
King Richard I doesn't have the biggest role in Ivanhoe, but he has two major things going for him as a protagonist. First, he is an actual historical person, one of the main figures in English history, which gives him a leg up on fictional Ivanhoe. Second, Richard is the reason Ivanhoe leaves his father to go on the Crusades in the first place. Without King Richard, we would have no angry-father plot and no rebellious-Prince-John plot. King Richard mostly operates in the background of Ivanhoe, but he also makes the whole novel possible.