>>665Tomobiki students have no idea that Ran is a psycho-bitch and she keeps up a pretty face in public. She is supposed to be psychologically damaged and holds a grudge against Lum, rather than being actually evil. So she's mostly normal, but then goes on her murder plans when Lum and Ataru are involved. So for them they're just attracted to her cutesy persona.
Ryu-chan also only sees the cutesy side. I don't think Ran and Ryu were ever in an adventure together with the gang, it was always one or the other.