The Opposite by Tom MacRae and Elena Odriozola

Nate wakes up one morning to see The Opposite sitting on his ceiling. This impish creature manages to ensure that whatever Nate says, the opposite will be true. Nate tries to fix each thing as it goes wrong, but the opposite happens! The mess he's making soon gets him into trouble and it takes some very quick thinking, and a clever twist, for Nate to be able to get rid of The Opposite once and for all.

All the Hallmarks of a Classic - Stephen Fry