Depends which ninjas you already have, if you have Darui, like EM or LM, you can save for 200 seals, and spend on cloud treasure during rebate. To get 3rd Ay, Samui and Omoi.
Otherwise, Roshi is quite a god ninja in f2p, useful in many ways, you c*e him with every main, you just need to make sure he doesn't die in first round.