Hi,
John is in Todd's corner. I believe that this idiom comes from the sport of boxing.
Think of this picture. In a boxing match, each boxer has a corner, where he sits between the rounds of the contest. Each boxer has a helper who stands at the boxer's corner to give the boxer water, fix his cuts, give him advice and encouragement, etc. This helper is said to be 'in the boxer's corner'.
Thus, John is in Todd's corner means that John is giving Todd support and encouragement.
Best wishes, Clive