When the Uni-Power chooses a partner, it endows that person (or, rarely, persons) with the powers, memories, and costume of Captain Universe. The partner can use the powers of Captain Universe until the peril is surmounted. However, if the partner tries to use those powers for personal gain or for evil, the Uni-Power immediately deserts them.
From: The Invincible Iron Man #4
From: Thors #1