Who is the fifth siner in yugio 5d's?

Answer 1

the 5th signer used to be rex goodwin (a guy with the mark of a signer and a mark of a dark signer on each arms) but after the other signers defeated the dark signers and yusei, jack and crow defeated rex goodwin crow (who was actually not a signer) became a signer after rex was defeated in a duel by getting yuseis old mark and yusei gettin rex goodwins mark

so in another way:

rex used to be a signer but he was defeated

so crow became the 5th signer

try and watch the series a bit more