"It was like... She was just an empty husk..."
So, if you're among those of us who pulled Okita at some point in FGO, you may have noticed what I'm about to say already, but I'll explain in full anyway.
In FGO, Okita's voice lines vary quite a bit based on what she's doing. Out of combat her voice is the typical cutesy type that Aoi Yuuki is known to play, but in the battle dialogue her tone of voice becomes much more serious, almost to the point that she sounds like a different person.
The line of dialogue shared by the MC and Izou that I quoted almost confirms something I've thought for a while based on what I said about her voice lines in FGO: in combat Okita shuts off most emotion so she can focus on the fight, but at the same time substantially altering her personality.