You can trigger the effects of these face-down cards by destroying them. Wild Tornado selects 1 face-up monster and destroy it. Dark Coffin makes your opponent choose between discarding a random card from their hand or to destroy 1 monster on their side of the field. And Statue of the Wicked lets you summon a Wicked Token on your field.
You can use Bluff Trap every time your Life Points decreases by 1,200. It lets you have a set Statue of the Wicked on your field which can be targeted by Greenkappa.
Using Shallow Grave allows you to re-use Greenkappa if you only have few copies. Special summoning it from your graveyard to protect you can sometimes force your opponent into attacking it. Useful to bring back if your opponent has multiple set spell/trap cards.
Is it optimal to have 2 copies of Kappa in a deck, or one is just fine? Btw, I have 3 Wild Tornadoes already. I don't like the silent magician so I won't reset because I have already 2 kappas (got lucky on guaranteed SR draw)
Target 2, not up to 2,so if there is only 1, this wont pop (correct me if im wrong). You can target your own as well, making wild tornado even better. But this is a fundamentally weak monster and will probably not be a good play after turn 1, especially in phoenix/red eyes meta, which they dont need much use for spell and traps anyways. This also cant target field spell like fire kings or umi, or counter naturia or ice barrier otk. Generally, a niche card, you have to flip it for its effect too so its not immediate. Not as good as some might think.
Red eyes mostly use red eye spirit and insight, and spirit is chainable to your effect to activate before you destroy. Econ and super rush may sometimes be run too, but STILL both are chainable. Insight obviously doesnt need to be set