"Ditto" Chess

This is another variant I invented. It is exactly the same as regular chess, but with an extra peice. I call this the "Ditto." It starts at the center of the board and whenever any peice moves, it moves the same way. Eg,

...peice and stops. Ditto can take white's peices on white's turn, and the same for black.

"ditto" cannot be taken, whenever something advances to take it, it moves away.

What if the ditto is in a corner, and I move to that corner. The ditto has nowhere to go. Would it be captured or would the move be illegal?

interesting.... I suppose you would take it....yes, that seems right.

Why put it on d5? That seems to give white a significant advantage in taking a pawn on the first move. Wouldn't d4 be better?

Im not sure where to put it. if it was on d4 then white wouldn't be able to open d4, so.... maybe it doesn't move for the first turn?

If it was on d4, white could open d4. The ditto would move to d6. Of course, if black plays d5, the ditto then takes d4.

Can the ditto cause check or take the king?

You are very quick at picking up these things. The ditto can cause check and take the king. Also, I have just decided that black chooses where the ditto is placed an the board at the start of the game. This makes up for the disadvantage of starting second.

What happens if you castle?  does the ditto not move? or does the ditto move the same way as the king when castling?

You say that the ditto starts at the center of the board.  So does black choose from the four central squares where the ditto starts?

I found a variant called Mimic Chess.  Although the mimic is a little bit different from the ditto.  The mimic does not block movement, and, for example, if the mimic was on d6 at the start of the game, 1. e4 would be illegal.  However this is legal in ditto chess.