Side
The leader of the shellfish has a knife in its side. It's fine though, don't worry, it's just a reminder that he won the great shellfish war back when they were trying to choose the new leader.
Picture the shellfish leader in your mind and make sure to pay special attention to the knife that's sticking out of his side. It's right in the groove of his shell.
そく
がわ, そば
None
Because of this knife in his side, he can't pronounce certain words correctly. The most obvious one is when he says 川 (かわ), but it comes out of the side of his mouth as がわ.
Try saying かわ out the side of your mouth like the leader of the shellfish and you'll probably say がわ too. Don't stick a knife in your side though, we won't pay your medical bills when you complain to us that this mnemonic got you injured.