What is Hestias weakness?
Hestia, as the Goddess of the hearth, was considered to be maternal, passive and non-confrontational. As for “cool emotionally”, I don’t think that applies, and it’s meaning should be obvious: Unemotional. There is no indication that she was considered unemotional. Also, you seem to imagine that goddesses must have a “weakness”, but that is seldom the case. There is no necessity for any god or goddess to have a weakness.