Dr Matthews is a doctor and surgeon who has appeared in a few episodes.
The Sensei offers them a new sub he calls the Sandwich Of Health, a meatball sub he claims is as healthy as a salad. Rigby is skeptical and says if anything it looks even greasier than a normal Sub. Sensei denies this and says he has one three times a day and is perfectly fine....only to keel over when his stomach explodes.
The Sensei is taken to the hospital where Dr Matthews says the reaction was perfectly normal given how incredibely unhealthy the sub was. And that given his 'size' he can't just get a normal stomach transplant. He needs a Death Kwan Stomach from the Death Kwan Hospital.
After being struck by lightning, neither Muscle Man or High Five Ghost are able to remember each other. They are taken to the hospital to be checked, where Dr Mathews tries to help.