Plankton, Tori and Cat Relationship

Ever since the girls stopped Plankton, he most certainly couldn't stand them but also he finds them fascinating as they are two humans that can breathe underwater, he did all the research but nothing came about until he found out by eavesdropping in a conversation that they have magical seashell necklaces that helped them breathe and walk around underwater without any diving gear, he once tried to take them off while they were spending the night one time at Spongebob's house but since it was impossible to take off due to the magic, he failed miserably.

When he found out that both occasionally sing and perform at the Krusty Krab, Plankton had to tried to persuade them to do the same for his business but since they obviously don't trust him, knowing who he is, they decline, making him extremely disappointed.

Plankton even sometimes tries to manipulate and trick Cat into telling him or leading him to the formula since he failed to try with Tori because she's not easy to manipulate and can see right through him of what he's trying to do. Cat annoyed him so bad that he didn't even bother to even get her to tell about the formula anymore, which was a waste of time anyway because she didn't know it herself.