Originally Naofumi was an extremely trusting and positive person, he was also enthusiastic about his situation when initially summoned, but this all changes quickly after he is betrayed by Malty. Left penniless, framed for rape, and hated, Naofumi becomes distrustful, cynical and spiteful towards the world. He becomes unable to trust anyone who is not his slave as a traveling companion and begins planning for events where people try to deceive him or rip him off. This betrayal has left him in an almost permanent state of anger and hostility, even while he usually acts calm and level headed on the surface.
His hatred for Malty is utterly without bound, to the point where he has become completely apathetic and ignorant to the advances of women in general, and even sometimes expresses disgust and hatred purely on the grounds that the other person is the same gender as Malty early on, though he never voices this to them. Due to Raphtalia and Firo, he becomes less inwardly hostile towards women over time, but is still evidently unable to notice their advances, Raphtalia herself, when her Slave Crest was being renewed, even asked him what he thought of her body, which had to be exposed for the application of the crest, only for him to completely miss the point. He was rather amazed when the Slave Merchant was willing to pay 15 gold pieces for Raphtalia after she became a beauty as well. He does however notice Atlas' feelings for him, if only because of how overwhelmingly blatant she is about showing them, and goes out of his way to have others he trusts sleep with him solely so they can kick her out of his bed when she comes by. Finally, when Firo goes into heat later in the story he was willing to do almost anything, including sacrificing the future queen herself, Princess Melty, to her lust instead, to protect himself. When his hatred was completely removed by the Wrath Dragon absorbing it, he did finally show awareness of Raphtalia's beauty, but the women around him found his suddenly happy and uplifting self so creepy that they threw him into the dragon's guts to get his hatred back.