In Single Player/Ko-Op:
Available in Versus Mode Only
In Versus Mode Find the Red Yin Yang in The Living Forest. In the area after you pass the Brotherhood of the Shadow you will see a small waterfall. Long jump over the waterfall and enter the doorway on your right-hand side. In this area find the first group of Clay Soldiers, smash one of their shields with the Fist of Ruin. Get them to follow you closer to the lower-right corner of the platform. At that corner you will slightly see a large stone pillar across the water stream. Throw the Clay Soldier with the smashed shield across the stream and into the pillar, it should come crashing down. Leave this area the way you came, and go back to that small waterfall. You will see the waterfall is no longer flowing, and inside is a Red Yin Yang that unlocks Reptile in Versus Mode.