It has always been obvious He isn't Saul,
But what We know is this:
1. He took entire Road Poneglyph instead of just making a Copy
2. He took it from Fish-Man Island that is Conveniently only Place among Four to be Affiliated with WG
3. Nothing during Fish-Man Island Arc suggested that They were Invaded or their Poneglyph taken by Force
4. WG doesn't warn the Public about him even though He is clearly Dangerous
5. He didn't try to Intercept Law or Kid or Luffy to take their Poneglyphs
It's clear to me that Shiryu's Theory is indeed Correct, this Guy works with WG in some way, shape or form,
He is a Guarantee that No One can find One Piece even if They make Alliances
Man With Burn Scar seems to be that Shadow Guy who holds a Sword similar to Shanks & probably same guy shown among Holy Knights,
If Garling is their Spandam (Rank Wise), then this Guy is their Lucci, He could be Mary Geoise Strongest Fighter (In History)