Tessa + MJ

I've been going through the series lately and I'm wondering how exactly Tessa and MJ are such close friends. They seem like very different people. MJ is very sweet and caring, while Tessa is very two-faced, selfish and she would often concoct schemes behind others' backs. Did anyone else find their friendship to be kind of odd? Since MJ is so different compared to Tessa, it's a wonder Tessa never took advantage of her good nature and pulled some con on her.