Hi! You should be able to use the "window" prefix, according to the documentation page (scroll down until you see "Styling text and windows"): https://www.renpy.org/doc/html/dialogue.html#Character
So, if you have a textbox2.png in your GUI folder for example, and you'd want B to use it, you'd do:
define B = Character("B", ctc = "ctc", what_prefix = "\"", what_suffix = "\"", window_background = "gui/textbox2.png")
You can apply your own style to this window by using "window_style", also mentioned in the same section of the documentation.