"I didn’t like the way she said that, as if Tyson were a big disease we needed to get to the hospital..." (Simile)
Here, we get the first sense from a character that something is up with Tyson beyond just being clumsy or awkward. Annabeth, who is somewhat judgmental but usually quite nice, has such a negative reaction that she treats him like a kind of disease that needs to be studied. It also shows how attentive Percy is to how people talk to Tyson, and how he's defensive of Tyson even in the presence of someone he's known for longer. Finally, the simile allows us a glimpse of how Annabeth's tone foreshadows her own difficulties with Cyclopses.
"built like the Abominable Snowman" (Simile)
Percy's early description of Tyson, which is juxtaposed with the revelation that he's actually quite timid, is probably our very first clue that Tyson may not be what he seems. It also demonstrates how much of a gentle giant Tyson truly is, with his exterior being threatening, but hiding a heart of gold, so to speak.
"Tantalus made a wild grab, but the marshmallow committed suicide, diving into the flames." (Metaphor)
Asides from showcasing Percy's sense of humor as a narrator, this also demonstrates exactly how extensive Tantalus's punishment is. Even the smallest portions of food remain beyond his reach.
"Chiron was like a second father to her." (Simile)
This simile, which is about Chiron and Annabeth, shows exactly how close the two of them are. For Annabeth, Chiron is far more than an activities director, meaning that his getting fired hit her especially hard.
"Like vultures, they’d been eating with their faces, and it didn’t look like they’d been feasting on Monster Donuts." (Simile)
Talking about the Sirens, Percy sees them for what they are: vultures. Asides from the comparison pointing out how they eat with their faces, it also highlights how predatory the Sirens are and how they live off of the anticipation of death.
