The Cloaking Crystal of Chrometheus is a rare magical object made of crystalline material and is supremely powerful and useful. It shields its possessor from any type of magic conjure, spell or attack; therefore making the user practically invisible and undetectable. It even renders potions ineffective.
In the sixth episode, Shinji had used it to secretly stalk Sabrina so that she would get in trouble with Professor Giest and would not be able to have dinner with him and his mother. The reason Shinji stalked Sabrina was because he was nervous about having dinner with her, which implies he does have a romantic crush on Sabrina.