So Leo leaves for a few days and Suya becomes a zombie. Does no else in the castle have the revival powers? He does have a good reason for stalking her then, other than the fact that she dies weekly. When Leo first started stalking Suya, it was out of worry for well... both of them really. Suya dies a lot and it's Leo's job to bring her back to life every time she dies. It just makes more work for him every time it happens or when she decides to make trouble. For example, in the bees chapter, she wiped out an entire race of demons to get honey because she was scared of them. Leo ended up having to bring them all back to life and then Suya was planning to burn the poor bees the next time she went to get honey. At the rate she was going at the time, Leo would have probably died from overwork. Leo's also one of the only people that tries to stop her when she goes a little crazy like the time she got drunk. That didn't really turn out well for him. Suya just does more and more dangerous things as time passes. We learn recently that she likes to do cartwheels on the railings near the magma. At that point, it would have been just easier to follow her and make sure she doesn't do anything stupid and make more work for himself.