The basic idea for this character creation challenge is simple. Someone posts two odd-couple character concepts and the phrase "together, they fight crime", as was once common in television and movies. Someone else then posts stats for those characters and another "they fight crime" concept for someone else to flesh out. Television and movies can be ripped off, but they can also be original ideas.
The inspiration for this thread was Steel Justice, an unaired cop show pilot.
He's a cop with a big gun whose son was killed by a bomb meant for him and he's the cops dead son reincarnated as a giant robot dragon. Together, they fight crime.
The size-changing can't be replicated by spirits or AIs, but both are reasonable explanations for the son's mind existing inside the giant robot dragon.