All Phantoms are former humans that were destroyed by despair and gave birth to their Phantom forms. The only ones who were able to fend off the Sabbath was Haruto and Koyomi, and then Mayu was able to fight off her own despair at the hands of her late sister. Those who are able to defy the transformation can become wizards and witches.
I'm really, really, really hoping that Gremlin's adherence to his human identity isn't another "I want to be human" plot like Ankh, Enter or Pinocchio. It does make the Wiseman's motives more interesting, since it shows that he may have some attachments with his former self.
The Infinity arc was subpar because it wasn't executed as well as Extreme or Cosmic. (Granted, the Cosmic arc was phenomenal and arguably the peak of Fourze's writing). It's also becoming formulaic so after the third go around of the protagonist suffering from a personal crisis and then overcoming it through conviction might get a bit old. Fourze's Cosmic arc was good because it was able to cap off not Gentaro's character arc, but Ryusei's character arc. It was able to integrate the final form toy spam into a meaningful arc for a secondary rider while also at the same time show off the culmination of the friendship theme of the first two acts. It wasn't just two episodes, it was also the end of Ryusei's arc so it definitely had more time to breathe and settle in for us, and thus was more climactic when Gentaro achieved Cosmic.
I appreciate the out of suit fighting on Shiraishi's part, and while on paper the story seemed good, the execution was sketch and it felt hollow, despite Koyomi's deteriorating condition. I was more impressed at how Nitou felt personal responsibility for WizarDragon's death and how Legion commented on how Haruto had changed Nitou. They didn't give him so bombastic arc like Fourze unfortunately, but he's pretty much on Team Haruto now.