Plankton is the "bad guy" of Bikini Bottom. He owns a restaurant, The Chum Bucket, and is insanely jealous of the success of the Krusty Krab. He is constantly plotting to steal the recipe for the treasured Krabby Patty. He is also the smallest character on the show -- he is actually smaller than a Krabby Patty.
Plankton would definitely fall into Kohlberg's 1st stage of moral development. He is only concerned with his own interests, and as a result has no friends.
In the episode titled Plankton!, Spongebob and the gang are first introduced to Plankton when they realize he is trying to steal a Krabby Patty. Plankton clearly understands the idea of reciprocity from the 2nd stage. In the same episode, he gives Spongebob a golden spatula and says to him, "Friends give each other gifts," trying to appeal to Spongebob's limited understanding of his evilness and his good heart. He eventually uses a mind-controlling device to get Spongebob to bring him a Krabby Patty. Luckily, his plan is foiled and all is well at the Krusty Krab again, but Plankton will never give up his fight to get his hands on a Krabby Patty one day.
