I'm playing as Lizardman. I block an attack, use the R on Drifter and play Sardine's Beach Special as Reversal. Can my opponent cancel it with Challenged?

Cards like Sardine Beach Special only have the keyword if you are playing as the specified character. In your case, you are not R. Mika, so SBS does NOT have the Reversal keyword on it. Your opponent may respond with Challenged.

So... the Reversal keyword isn't printed on the card then?!

nope, he's got it right.

the keyword is only there if you are the character.