When you select "Serpent-End" in the conclusion tab of the journal and then close and re-open the journal, it changes to "Cookiesaki Lady". This makes it impossible to correctly identify the Serpent-End.
Also, the Ghostpedia entry for Serpent-End does not show up when you open the journal outside of a mission.
Why doesn't he have any dialogue? And who is the one that the NPC on Sunflower Street said you could visit at any time? On that map.