I think it's just Aspasia seemed to handle Siofra as easily as she did that made me think she's OP. I tend to not use her, choosing to stick with Metatron, Sarah, or Celica (depending on situation) but though I'd give Aspasia a chance and was thinking "geez that was easier than I thought it'd be!"
Also, outside of Metatron, only Celica really says anything about Maya headpats. Everyone else doesn't say anything when the option to headpat Maya occurs. Maya needs more headpat love!