Squidward

What does he usually think about?
About peace and quiet. He's constantly irritated by the noise and frantic energy of SpongeBob and Patrick. Something like, "When will they finally calm down?" often runs through his head.
About his creative side. Squidward considers himself an artist: he plays the clarinet, draws, and dreams of recognition and the big stage. His thoughts are filled with hope for success, but they're almost always mixed with disappointment that no one seems to appreciate his talent.
About a job he hates. He's a cashier at the Krusty Krab, and he finds it humiliating and boring. In his mind, he often contrasts it with his lofty artistic aspirations.
About the neighbors. SpongeBob sincerely considers Squidward a friend, but Squidward himself sees him more as a source of endless problems. His thoughts about SpongeBob are a mixture of irritation and (sometimes, almost imperceptibly) quiet affection.
About self-respect and status. It's important to him to appear refined and "above" the bustle of everyday life. He often thinks about how to emphasize his uniqueness and distinguish himself from the "ordinary."
How does this manifest itself in the lines?
In the series, his inner world is clearly visible through his phrases: he sighs, makes sarcastic remarks, and then dreams out loud. For example, his sarcasm and tired comments are precisely his voiced thoughts: "I can't take it anymore...," "This is simply unbearable...," "I'm an artist, I deserve better!"

Dissatisfaction with his surroundings. His biggest pet peeve is his noisy and carefree neighbors, SpongeBob and Patrick. Squidward finds their behavior frivolous and annoying, though they don't even realize it and sincerely consider him a friend.

Job aversion. He works as a cashier at the Krusty Krab, but he loathes the job. He's irritated by both his greedy employer, Mr. Krabs, and the very need to constantly interact with SpongeBob.

Dreams of a different life. Squidward dreams of something bigger: becoming a famous musician or artist, getting rich, realizing his creative potential (he plays the clarinet and draws). But he usually fails to realize these dreams.

Feeling misunderstood. He often feels undervalued: others don't see his talents, yet he's unable to honestly acknowledge his own shortcomings. He tends to blame the outside world and other people for his failures.

Internal contradictions. The paradox is that, despite his grumpiness and even bitterness, Squidward sometimes wonders: what if life really would become empty and boring without these noisy neighbors?