Yes this is the one. While at first glance perespero seems to be the answer , but i thought at that time that since marco perespero alliance existed for just 1 chapter & fizzled out so this is not oda's way or style to foreshadow or hype some mystery , so it had to be something that Oda wants to drop a hint for now which will play a large role in the story later which perespero clearly didnt.
The way i see it is that Marco went to investigate the strange shadow (being introduced this chapter) & ended up finding Perespero who had now climbed the waterfall.
Even if we see the chronology of events :- Marco knocks out perserpero BMP ship & moves on towards onigashima-> since he is now on his way to onigashima so there is no way he saw perespero's shadow because for that persepero had to be instantly back atop the waterfall -> So while perespero is still down Marco spots a shadow while on his way to onigashima -> reaches onigashima & goes back to inspect the shadow & by this time perespero has climbed up.
Let me draw this:-
M <- Marco
P <- Perespero
BMPS <- BMP ship
WF <- Waterfall
O<- Onigashima
GS <- Giant shadow
Chronology of events:-
1) O-----------------------------------------------------WF (M encounters BMPS & P & knocks them out)
2) O--------M(on ship to O spots GS)-----------------WF(P is trying to get back so it cannot be his shadow that M sees in all likelihood)
3) O(M reaches O & goes back to GS)---(P is back)--WF
4) O------------------------M & P meet ----------------WF --(BMPS still not atop the WF i guess lol)
:P
yea we need the reveal asap
