Mortimer Chauncey "Morty" Smith Sr. of the Prime Dimension is the titular deuteragonist of Rick and Morty. He was often incorrectly identified as Morty C-137 until Season 5.

Morty is Rick Sanchez's grandson and is often forced to tag along on his various misadventures as a sidekick whilst attending Harry Herpson High School along with his sister, Summer.At the beginning of the series, Morty was shown to be an incredibly anxious and submissive teenage boy. He had little say in the adventures he went on with Rick and was often verbally abused and manipulated by him. However, Morty eventually gained more confidence as time went on and is now more capable and independent rather than a mere sidekick. This newfound confidence also allowed Morty to distance himself from Rick more often due to his despicable tendencies, although Morty still stays by his side when necessary.

It has since been revealed in "Solaricks" that Rick (C-137) is not his true grandfather, as protagonist Morty is actually the biological grandson of Rick Prime, a version of Rick who murdered C-137's family. After decades of failing to get revenge on Rick Prime, Rick eventually arrived in Morty's dimension to take his place, kickstarting the series and thus Rick and Morty's adventures.

Morty, despite being fourteen, has a mental age of several lifetimes. This is due to living in a time freeze for six months along with Rick and Summer, playing fifty-five years inside of the arcade game Roy: A Life Well Lived (the game is designed to cause memory blur after a play session, meaning Morty only has a vague recollection of this life), living for several more months inside of the Teenyverse, an especially long video-game like reality save, and growing up for several decades as part of a manifestation inside of the Fear Hole (the Fear Hole as an entity is also designed to cause memory blur related to an individual's experiences within it).