Green’s Dictionary of Slang

(orig. US) a promoter, a publicity man, a morale booster.

1934		Harold Arlen [song title] You’re a Builder-Upper.
1948		in Forbes LXI 39/2: As Gimbel’s head man, it’s my job to be a builder-upper, to strengthen the morale of my team, never to reprimand anyone in public.
1991		F.E. Mason in Examiner (Chattanooga, TN) VII:1 🌐 It is always good to let a man leave you with the feeling that he is a little taller, not a little smaller, than he was before you met. Become a builder-upper, not a tearer-downer of people.
1997		Christian Community Churches Inc. 🌐 The president isn’t only the leader and spokesman, he/she must be executive, chairman, discussion leader, builder-upper, motivator, investigator, arbitrator, referee, politician, and statesman.