I read this book as a young child in my grandmother’s house so it must have been published prior to 1950 ( late 30s?- 49?)  My sister thinks it was called Piccolo, but that title brings up no hits.  It is the story of a young boy who get a little “person” good luck charm who helps him. ( I think bullying was a theme back then too!)   In one illustration the little charm person is seen in his shirt pocket when a photo is taken of the boy doing something heroic, I think.   Along the way, the little charmer meets up with a cockroach who is going to attack, but the little guy puts a sewing pin in him ( I know weird)  but they become friends with the little boy, who paints the cockroach bright colors and he becomes the leader of his group winning against his adversary because of the paint job and pin!

The good luck little person ends up leaving because the boy doesn’t need him anymore.

It was an oblong book with plain blue/grey sort of cloth cover and had “flat” illustrations.  Another illustration showed the cockroach all painted different colors.