I have been a SpongeBob Squarepants fan for quite some time. In that regards, I was thinking of drawing an OC that is a ninja dolphin named Seiji. Before you burst your guts laughing, there actually WAS a river dolphin called "Ninjadelphis" found near a ninja training center in Japan. I think it would be ideal since Sandy and Spongebob know karate.

SpongeBob Squarepants (C) Stephen Hillenburg (RIP)