Because that is Tori's ace in the hole currently with the heroes. It makes tech of hers harder to reverse engineer because she's the power source, and near impossible to steal and use against her for the same reason. It has already come up in story that heroes presume that she's vulnerable within the suit rather than varying levels of insubstantial flame, and in the case that Tori has to ditch the suit she isn't unarmed.
Basically, the heroes have no clue about half of Hephaestus's power set. That being said...
She is living flame after all, her being as vulnerable as she is to water, and possibly needing more oxygen wouldn't be known by the heroes to avoid when trying to non lethally bring her in.
Ivan's daughter is described as having an incredibly kind heart and having the ability to manifest incredible sharp blades when stressed. Kristoph is incredibly kind to children and manifests razor sharp blade wings. Kristoph may have come into contact with Ivan's daughter and either deliberately or accidentally 'infected' Ivan's daughter. Ivan has said that he has introduced his kids to a few members of the guild. While Ivan didn't specifically mention Kristoph, Kristoph seems a very obvious choice of a person Ivan could trust to deliver a message or aid to his children.
