Hazama was always his own person. Relius made artificial bodies for Terumi to inhabit. We see two of those. One is Kazuma Kval who was used in the Dark War. He's the protagonist in one of the Phase Shift novels. Next is Hazama. These bodies had their personalities to begin with. Kazuma uses the Boku pronoun. Hazama uses the Watashi pronoun, and later starts using Boku in BBCF. Terumi uses Ore-sama. Everytime Hazama talked in extremely vulgar way in the Japanese, that was Terumi taking control, mostly obvious with the hair spiked up. Hazama in the Japanese version speaks very formally and in a polite manner.
Same yet "different" at the same time (If that makes sense) I've always seen Hazama and Terumi as "one and the same" even if Hazama and Terumi are different personalities that share the same body, they'll still be the same in a sense. Think of it like Inner and Outer Moka from Rosario Vampire
