Alternative hypothesis: the First Finger/"oldest brother" (Huetoto?), who can erase the presence of seven different people (Six Fingers+Kirio) from people's memories and physical photographs (and presumably video recordings, if there are magical security camera equivalents) on a massive scale (easily dozens-to-hundreds of people, possibly thousands, multiple photos, and probably days of video, depending on how long they worked at Walter Park), who looks remarkably like Purson, with a similar hair color and similarly ridiculously simple facial features...is actually Purson's missing brother.
When phrased properly, it's a form of magic at least a conceptually adjacent to Detection Warding (erasing your presence), conceptually, and we know from Clara's Toybox that fusing Bloodline Magic with other magic (illusions, in her case) is possible, and can produce entirely new effects not linked to either original magic (like draining the magical energy from those she draws inside her little world). If his brother can stretch Detection Warding to literally making himself/Soi immaterial, able to pass through material objects (which he did in that flashback, we see Soi trying to touch his mom and failing to do so), it's not a huge leap to think he can pull off the whole "we leave absolutely no traces, not even in memories" thing that the Six Fingers are infamous for. That seems a lot simpler to me than a secret seventh Finger. YMMV.