This page is part of the vintage Canon collection I maintain at canonfd.org
The Demi (French for "half") was a half frame camera with a 28mm f/2.8 that uses ordinary 35mm film but takes two 24 x 18mm exposures on a normal 24 x 36mm film frame.
Canon rode the half-frame wave and craze after Olympus introduced its Pen in 1959 and manufactured several Demis, including the C: the original Demi (1963), the Demi S (1964), the Demi C (1965), the Demi Rapid (June 1965), the Demi EE17 (1966) and the Demi EE28 (1967). By that time both Canon and its competitors were looking to make inroads into the SLR scene, possibly explaining this compact format's demise.
This was however the only to feature interchangeable lens and the set comes with a f/2.8 50mm lens. Good quality versions of these are hard to come by, so I ended up buying this online at auction. When it was first sold in 1965 it went for 18,000 yen. (Source)