i think the first episode that we see lisa is when dudly likes her and he cant speak to her so arnold tries to help him. So she started out nice where arnold liked her and dudly so was she always ment to be nice then they decided to change her into someone they didnt like and was mean or like did they decide to change how her character is or what?

Yeah that was weird but maybe she was always mean she could have had two personalities, i can guess is if she went on dates with dudley and he broke up with her she would start to treat dudley and arnold badly because she is still mad that dudley broke up with her girls are known to do that lol

http://www.youtube.com/watch?v=RxVfiYYo0Nw
http://www.youtube.com/watch?v=0KflQ5--uSY
http://www.youtube.com/watch?v=RGJRkTIZAmQ