Daidōji's outfit is in the sytle of Japanese middle and high school delinquents, also know as 'Banchō'. Her outfit is also very similar to that of Jotaro Kujo from the manga JoJo's Bizarre Adventure, down to the cut-in-half hat. A lot of her moves and combos are also reference to the manga series.
In the Senran Kagura: Official Design Works art book, it's stated that Hikage chose to be tattooed on the spots where her surrogate mother, Hinata, was fatally wounded.
Yagyū's name and eyepatch are likely meant to be a reference to Yagyū Jūbei Mitsuyosh, a renowned swordsman from Japanese history, who was said to have worn a sword's arm guard as an eyepatch.
In an interview with Marvelous Europe, series producer Kenichiro Takaki stated that his favorite character is Hibari, because she was designed around Takaki's interests instead of being made to appeal to different groups of fans like the rest of the cast.