That's because Tokugawa was a highly reserved man, overly cautious and fearful(?) until he was given enough power to start supporting Nobunaga openly and eventually he grew more balls and continued Nobunaga's ambition to unite the nation. Depicting him as a shy girl at start of the show seems a practical course, I seen Tokugawa's female depiction in other shows to be the same way, cunning but when it comes to confrontation, not very strong or stuff.
It's a standard depiction as over the course of time the female in another show also gained more confidence so I wouldn't worry about it.