The company is most likely based off the real-life manufacturer Toyota since most Karin cars are inspired by Toyota models.
However, the Sultan RS and Korean Mob Sultan, including the original Sultan, are modified like Subarus even though they are both based on the Toyota Altezza (also known as the Lexus IS300); and the Intruder is based on an Infiniti model.
The name might be a parody of Kia as Kia and Karin are spelt similarly and are both girls' names; the logo also bears resemblance to their corporate logo (mainly the top left tip of the "K").
In-game, however, Karin does not reference Kia Motors, as Karin vehicles are mostly based on Toyota vehicles.
Karin is pronounced as kareen by a Karin sponsor on Fame or Shame, but it is pronounced as "Karen" during the advertisement for the Dilletante featured in GTA IV.
The name might also be a reference to Toyota's Carina series.
There is a concept car by Citroën called the Karin.
The Karin logo is similar to the Circle K logo.
The Intruder is the only vehicle that is not based on any Toyota/Lexus vehicles, however it is also shares numerous styling cues with the 2002-2006 Toyota Corolla (or specifically, the Toyota Mark II in Japan).
The Sultan, despite taking up resemblance to the Lexus IS/Toyota Altezza, resembles more of the Mitsubishi Lancer Evolution VII/Subaru Impreza WRX due to the Sultan's AWD setup (as opposed to the Lexus IS/Toyota Altezza's FR drivetrain).