Boomerang is an agent with U.S. Army Intelligence.
His real name is Lloyd Raleigh and he holds the rank of captain in army. We learn on the first panel that he "pledged ... to eradicate scourges of modern civilization" and he eagerly accepts his first assignment. His mission is quite straightforward as explained by his boss at Intelligence when he says, "in our desire to free the oppressed world of sadistic killers we must eliminate them one at a time." So Boomerang is really an assassin.
It on his first mission, one takes him into Berlin, flying a captured German plane and speaking German like a native. It also gets him thrown into prison and it is there that he takes a moment and changes into his costume (no word as to where he had hidden the outfit). Suddenly, one of the other prisoners calls out his codename, "Boomerang!", in surprise. Keep in mind that this was his first day of his first mission flying from England to Germany and already those locked up awaiting execution knew of him. That is impressive.
Not so impressive is his outlandish outfit. Very short shorts (really a Speedo before its time), a torso-tight tshirt, a waist-high cape, swashbuckler boots, thick gloves, and a big floppy hat complete with a feather. All either black or green in color. And draped around him is a strap holding a small supply of boomerangs which he can throw with deadly accuracy. His fellow prisoners do not seem the least bit surprised at his change of clothing, though.
During his first adventure while he was in a prison awaiting execution, he meets a fellow inmate, the lovely British citizen Diana. What she was arrested for is not known but what the nasty bad guy holding her intended was not so mysterious. She was present when he changed into his Boomerang garb so she is aware of his identity. From his second adventure on, Diana opts to join him and seeing that she was England's bow and arrow champion before the War, she turns out to be a pretty lethal addition to the job.
The second mission has Boomerang able to impersonate a Japanese pilot, not an easy task. It also has him flying a Mitsuibishi aircraft and speaking Japanese as well as he spoke German before. Of course, his ruse is discovered through no fault of his own and he is about to be killed when a arrow comes whizzing from nowhere and Diana, who had snuck aboard the plane, dressed in a costume even more eye-catching than Boomerang's, calls out for him to "come forward". When he does, he has whipped off his outer garb and assumes that of Boomerang (where the hat was hidden is not explained).
Raleigh and Diana (we never learn her last name) are in California when the third recorded adventure takes place, having a bit of R&R which, apparently, they need. For the fourth and final romp, they are in Burma. (As her cover for going wherever Captain Raleigh goes, Diana takes the job of a war correspondent.) They certainly get around.
As an action team, Boomerang and Diana (she never gets her own codename) make a pretty good team. He is most definitely the brawn and she is without question the brains and yet he gets top billing. She gets the spiffier costume.
It is at the end of the final adventure that we learn Raleigh has been given orders to head elsewhere and he and Diana must part company. She says she will wait for him to come back and perhaps Boomerang and Diana will again see action. That has not yet happened.