The worst thing there is that they had an entire two parter to establish that Phantoms are completely unredeemable, but then just drop that with no explanation for Wizardragon and Chimera.
Chimera just lacks explanation period.
Where did he come from? Was he a wizard that turned into a Phantom or were Phantoms natural creatures at some point? Was he captured willingly by a wizard who struck a deal with him or captured there to be used? Considering he's
Chimera, could he have been man-made? Hell, were Phantoms
truly irredeemable? The villains we know are all, technically, newborn Phantoms for the most part, but Chimera existed long before the others were forced out through the ritual. What if a Phantom
could be redeemed, but the ones we knew were the ones that couldn't be? It's not like anyone really
tried - what if our opinions of them were colored unfairly by psychopaths of the race, and we were being no different than someone who decides a whole race of people is a certain way because of outliers? Really, thinking on it a little, Phantoms could have been made into something rather tragic themselves - they're sentient beings clearly living inside of people, but can only be born at the cost of their host. Imagine if a Gate learned this and developed a real relationship with their Phantom or something - what if the Gate tries to off themselves for the Phantom, and the Phantom ends up being born not actually wanting to have been? There's a subplot for you, Toei.
Backtracking, what I was starting at getting at is that was while this explanation definitely may have been lacking in Haruto and Dragon's case, could it be it never was in Chimera's? Mainly because he/it was lacking so many more explanations to begin with - from what I'm remembering, at least his/its personality never changed as drastically as Dragon's did - he and Nitou always knew what was up. The difference there, of course, may just have been that Nitou wasn't actually his Gate.