River - a powerful but volatile psychic with largely untapped abilities. Unless you have particularly strong shields, she'll be able to catch thoughts, images, and emotions from your mind, often without realizing that she's doing so. Any ability to shield her own thoughts - when she can access it at all - is as unpredictable as the rest of her brain, though she does have the potential to develop it further. So telepaths can read her in return, but her mind is an intense, chaotic place, a mixture of faster-than-light thought processes, traumatic and violent memories, and incoherent nonsense. She might or might not know if someone else is in there with her.

Ruka - He didn't think that anybody at Ohtori could literally read his mind, but, well, he wasn't certain that they couldn't, so he has enough rudimentary ability to hide his thoughts so that they don't leak all over the place. Thus, while he's also not immune to telepathy, he's not an open book, and - like River - could probably develop much stronger shields with a bit of training. While intuitive, he has no psychic abilities of his own.

Andrew - Normal human with no mental defenses whatsoever. Have fun.