Invent the new worst/most useless chess piece ever!

The Chernobyl: moves one square every turn, can't be taken. All directly neighboring pieces must move away within three turns or are irradiated to death. (friend or foe)

I have done that in my P.E

because nobody in my school likes me

The introvert. It can only move (teleport) to open corner squares (a1, a8, h1, h8), takes up one of your rooks, and cannot capture anything. If it is captured, your opponent's introvert also dies, presumably because everyone else it would hang out with would try to talk to it, and it can't survive in such an environment.

The Puppet Master: It controls all other pieces, even those who would be king.

life would b♭ without [insert instrument here]

agreed

The Politician - two for each side plop themselves on d4 and e4 for white, d5 and e5 for black, and filibuster the center. It cannot move, cannot capture pieces, can only be captured by the opposite sides king, and knights cannot jump over them. Good luck getting anything done with them around!

The Blocker

Can be placed anywhere on the board where there is not already a piece, cannot capture or be captured. Therefore it essentially "blocks" a square and renders it unusable by both sides. Can be jumped over by knights however.

I actually feel like this could be interesting, if not useful.

The Trump

Simply the greatest piece ever, believe me

The Booby Trapper

This piece, which looks like the bird, the boobie, traps any of your own pieces that move on any square next to The Booby Trapper. Opponent's pieces are unaffected by The Booby Trapper. The Booby Trapper gets set up on e4 for white at the start of the game, or e5 for black. You are not allowed to move any piece trapped by The Booby Trapper until you have moved away The Booby Trapper. If your king happens to get trapped, and you get put in check, you must get out of check by either blocking or capturing the piece that put you in check, as you won't be allowed to move your king out of check while it is trapped. To move The Booby Trapper away, you must have your opponent capture two pieces or pawns first before you will be allowed to move The Boobie Trapper one square in any direction. The Boobie Trapper is not allowed to capture any of your opponent's pieces, and cannot be captured by your opponent, so it sits there the whole game hoping that some of your pieces will fall into its trap. If you are prevented from making any legal moves because your only remaining pieces are trapped, you must pass your turn on to your opponent.

This would get seriously annoying very quickly. 

Knights who say ni - pretty useless!

You may promote one pawn into a mother-in-law and automatically the opponent's queen turn against her own king.

That is a GREAT! picture, the guy has a real sense of humour, crossing out the work 'work' and substituting 'move'.

I became a little fazed with panhandlers in the 1990s when I saw in Paris one guy with a bottle of Champagane half hidden behind him and another with a mobile phone, (in the days when they were very expensive).

I know there are no doubt many deserving cases but these were not.

The CIA agent, when it captures a piece it uses it as a pawn.

the othellonator.  X2 on the board.  When pieces of opposite colour are sandwiched between two othellonators, they change colours.  if one is captured they both die.

What if it moves to the king square? Is it checkmate?