If you don't want to complete the campaign to unlock Ratchet for Multiplayer Mode, there is another way to do it. Go on the Unlockables screen, then press Up, Right, Down, Left, Up, Start. Ratchet will be made immediately available.
Complete the indicated task to unlock the following characters in multi-player mode.
To unlock PlayStation3 trophy rewards, complete the following Trophies. Wish granted!
Aside from the default Transformers, you can unlock two additional characters for Multiplayer Mode. You can unlock Shcokwave and Ratchet by completing the campaign.