It's a Bikini World (1967)

As soon as he sees her for the first time, Mike Samson, the big man on his southern California beach and the object of desire of any girl on the beach, knows that he wants Delilah Dawes, who is in the area visiting their mutual friend, Pebbles. Despite Pebbles' assertions that Mike is a great guy, Delilah finds him arrogant and conceited, and flatly rejects his advances. After Mike secretly overhears Delilah tell Pebbles that she would rather date a modest, serious boy who doesn't necessarily have to be a star athlete like Mike, Mike decides to conduct an experiment to see if Delilah is true to her word or full of hot air: he will masquerade as his serious, quiet, studious, uncoordinated and bespectacled "brother", Herbert, who in reality does not exist in any form. The ploy works as Delilah starts to date Herbert in order to bring him out of his shell. Mike's ability to pull off the masquerade is hindered by his roommate/Pebbles' boyfriend, the dimwitted Woody. Complications ensue when Delilah questions why she never sees Mike and Herbert at the same time, when Mike starts falling for Delilah despite his initial intention purely to show her up, and as Delilah starts to fall for Herbert who doesn't really exist. So if/when Delilah finds out the truth, will Mike still get to keep the girl?