how can she not tell from her voice tho.
Like it's a manga, so I can suspend my disbelief to an extent but,,,, It would literally be a dead give away.
There are people who sound like they could be either gender. You could also say that since Ookuma believes Kanda to be a guy she subconsciously hears her voice as being more masculine than it may actually be.
Actually I was so, even though I was a girl but I had a fairly heavy voice and my face wasn't the type "Ikemen" but I can look like a guy hanging from my hairstyle and my dress way
