It really could go several ways but I would stat him as an Ally with a low frequency of appearance.
- He is Extremely loyal and capable, beynd most hirlings
- He does go into the field sometimes to rescue, deliver or otherwise aid batman
I would still pay him out of Cost of Living and that handles his more mundane appearances with the Ally Appearance rolls being only exceptional circumstances.
I might also just han him as a stand in for Serendipity instead of Ally.