Rogue didnt start out as a scared & whiney brat (aside from the initial issue of her turning up on the X-men door step, which was understandable. She was always cool as a cucumber and dependable/resourceful in a fight even in the early days- which accounts for her early popularity. Rachel Summers on the other hand... And for pure brattishness=Jubilee.
Well maybe 'whiny brat' was a strong term. She was cool and dependable in a fight but a very childish character when not... like I say albeit in a likable way.
She did spend a lot of her time feeling sorry for herself and throwing tantrums, particularly during the period when the Carol/Rogue personalities started to separate and battle for control.
Also her answer to problems was to run away... something that continued well into the nineties.