For the Agatsuma thing something amount the lines of "If i tell you i'm gonna have to kill you, and if you open your mouth we would know i promise", or if he wanted to say the true but keep it short, something like "My organisation is going to clean your memory if you keep on getting your nose into this, i'm not kidding, if you ask your friends what happen at school, they won't remember a thing" or if he wanted to keep the alien bull crap "I have imnunity to the alien virus, but you might die if you get to close to them"
This are just examples, but i just found annoying how much the mangaka tries to make lies bad, offcourse things would go terrible wrong is the best lie you can come up with is crap like that :S^