Can confirm he is there from the beginning, but I also didn't notice him until after I'd beaten 4 or 5 bosses and was looking to do something with their souls.
It turns out I missed an entire boss fight as well! It was one of those times when I thought I had checked all the hallways and I knew right where to go but saw some other path and followed it through instead, never to return. Whoops! I probably spent a good hour before NG+ just wracking my brain trying to think of areas I'd seen but not gone to, and eventually decided I'd get them all next time and just started it up.
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