In Mrs. Doubtfire, the father who creates the disguise is named Daniel.
Daniel Hillard is played by Robin Williams. At the beginning of the film, he is a talented but unreliable actor whose marriage to Miranda has ended. After a custody decision limits his time with his children, Daniel invents the housekeeper persona Mrs. Doubtfire.
The disguise lets Daniel work inside the household and see his children regularly, but it also creates escalating complications because Miranda does not know Mrs. Doubtfire's true identity. The plot uses this deception for comedy while keeping the parent-child relationship at its center.
The character's full name is Daniel Hillard. “Mrs. Doubtfire” is the assumed name, not the father's real name, which makes Daniel the precise answer.