Its a Model 1928

Garand thumb

Theoretically, she uses a Thompson M1928.

In reality, the model she's holding (in her artwork) is a hybrid between like three or more different models. The overall appearance and handling properties would be correct for a M1928 variant. However, it's operating system (Straight blowback as evidenced by the side charging handle, rather than the iconic top-mounted one of the guns that operated under the Blish principle), sights, and other details (particularly noticeable around the magazine well area) indicate that it is an M1A1 receiver, which would mean that it shouldn't be able to utilize drum magazines. The barrel length is not correct to any historical model, and is instead reflective of modern reproductions of the gun which need a long barrel to be compliant with current legislation (At least U.S. ones), should they want to avoid being classified as SBR's.

However, all of that wall of text was probably for naught. The mish-mash of features, alongside the acknowledgement of the reality that the dev team almost certainly did not have access to reproduction Thompsons, nevermind real vintage ones, is a near dead giveaway that the model was primarily based off an airsoft 'M1928'. Quotation marks because the airsoft M1928's are M1A1's with M1928 barrels (alongside the Cutts compensator), sights and drums. The oddity about the model held by Thompson both in her chibi and artwork, is that they employ M1A1 sights (not M1928A1 because those lacked the protective triangular sheet metal wings) and lack the Cutts compensator, which IIRC while not unheard of for earlier designs, it was basically a staple of the Gangster-era Thompson. The long barrel was almost certainly implemented for artistic reasons so to look more intimidating.