Actually, during the time when the Mechanical Heart was sealed, Dorothy could have forcefully signed a Familiar Contract, but after giving it some thought, she eventually chose not to do so.
After all, this Mechanical Heart wasn't like Buster or Fafnir, whom she had personally nurtured from the start. Zero already possessed a certain level of intelligence, and forcing a contract would only intensify the contradictions between them, leaving hidden dangers in their future interactions.
Dorothy certainly didn't want her own familiar to stab her in the back later. That would be too sad, and it also went against her initial intention of having a harmonious family.
So, she planned to try and persuade Zero to willingly enter into a contract with her.
Of course, this wasn't going to be easy, and she wasn't very confident about it, yet she really wanted Zero as her familiar, especially since its innate qualities were just too enticing.
