Jekyll and Lanyon are alike in being male, from privileged classes, and interested in science/medicine. They differ in that Jekyll is drawn more directly to experimentation, and to, forgive the cliché, the dark side of things. He is mysterious. Lanyon is, by contrast, cheery, even sunny. Lanyon is open; people understand him. He's a positive sort, at least until he observes the transformations that are happening. That depresses him, and breaks his health. However, Lanyon's sufficiently drawn to the dark and experimental that he's willing to watch Hyde drink the potion.