Shirasaya Iyo

Position of the mark: On her back
Curse:

Spoiler

Forbidden to use (speak) proper nouns, especially names

Gift:

Spoiler

Mastery of using any tools and gadgets

With those glasses and extra large boobs, she looks like the oldest among the cursed people even though she is actually not. She has a strong sense of righteousness and decency; that's why she remains as a normal schoolgirl even though she has the ability to be the world's greatest hacker.
Her circumstances with respect to both of her curse and her ability makes her somewhat the most fortunate on the surface among the cursed people. The burden she is carrying is actually as severe as those of others. Her restraints makes her awkward in dealing with people especially in conversations. But that has nothing to do with her not being able to "read the atmosphere" and butting-in off-topic.