I do not think Kansuke is Rum (The first suspicious person is never right!), but I do think that Kansuke is being used as a model to show off how to properly hunt for Rum. Conan used Kansuke's appearance to point out how various physical features satisfy Haibara's description of Rum. Kansuke is a strong young man, but if he let his hair down he might look feminine, and he uses a cane which makes him look elderly. We should be prepared to do the same for other characters who might appear: examine how their physical attributes and actions might make them look like a strongly-built man, feminine, and elderly. Also keep in mind we know Rum uses a prosthetic eye, so we probably want someone with two eyes like Superintendent Matsumoto. (I'm only picking on him because he injured his face across his eye and might have a prosthetic. I don't see a way a mountain like him could be mistaken for feminine.) Files 328-330 (Sato's Omiai Case) are probably helpful to review.