The first thing Arnold did after coming out of the portal was to visit Nina. It had been a while since he saw her and to be honest, he didn't like the thought of leaving her and Nicole without any protection in his absence.
Nicole was more than enough to protect Nina but still, as strong as she was, even she couldn't notice that Nathan was possessed by someone. And Arnold didn't want to take the chance again.
He was worried the Paladin or Vassal who attacked Nathan could also harm others thus he did what the system recommended him to do. He accessed the shop from his interface and bought two items, a necklace and a ring. He got the ring for Nina while the necklace was meant to be given to Nicole.
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OBJECT: Crimson Witch's Ring
TYPE: Accessory
GRADE: Cosmic B
EFFECT: INT +150, Provides complete protection from mental invasions.