mirror making character split

hello blendswap i have a question to about mirror.Why is it that when im in object mode with my model the model spilts when i scale it on the x axis and z axis.Also i notice that the model have planes at the back of it and when i delete one of them the model splits and connect again at a wierd angle?I anyone knows how to make it that when im in object mode with the model and make it that it doesnt split.Can someone please tell how to do so

In object mode use Ctrl+A to apply its rotation

The plane is used as the Mirror Object in the mirror modifier settings. It is set as the centre point of the mirror rather than the object origin