Same chin, same smile, same head shape, one is ashame of his head nut, both smoke
We haven't seen under Shiryu's hat either...
Maybe Usopp's observation haki will be key or tested vs Shiryu?
Shiryu's ancestor was framed and executed for something that wasn't his fault, labelled a traitor and liar. Shiryu now worked for the WG before, but executed some "innocent" guards without "meaning to" and apologised for it.
Idk where I'm going with this, I'm just crack theory shit posting lmao.
I've suspected Usopp could be a Montblanc and or a Marshall like Teach too though. If Shiryu turned out to be a Montblanc, that would be VERY surprising/unexpected and interesting.
I used ChatGPT to come up with a backstory for him in relation to Noland at least and thus why Shiryu worked for the WG in Impel Down and then why he joined Blackbeard too, on a personal reason/for his own agendas:
Before becoming a notorious jailer at Impel Down, Shiryu hailed from the distant lineage of the Montblanc family. His full name, unbeknownst to most, was Shiryu Montblanc.
His family once lived on Jaya, where they were related to Montblanc Noland, the great explorer. After the Knock-Up Stream catapulted half of Jaya into the sky, forming the Sky Island of Skypiea, Shiryu's ancestors remained on the Blue Sea. They carried the legacy of Noland, weaving his tales into family lore, while maintaining a sense of honor, courage, and thirst for exploration.
As a child, Shiryu was fascinated by the tales of his ancestor Noland, especially his unfortunate encounter with the World Government that led to his execution. This seeded a sense of resentment against the World Government in young Shiryu, but also instilled in him a strategic mind - he realized that to fight the system, he needed to understand it from within.
Shiryu joined the ranks of the World Government and was eventually appointed as the Head Jailer of Impel Down. Working from the inside, he had the opportunity to understand the darker side of the World Government, reinforcing his resentment. However, the brutality and ruthlessness of Impel Down gradually influenced him too, hardening his once noble heart.
When Blackbeard arrived in Impel Down, Shiryu saw a reflection of his ancestor's rebellious spirit in him. Shiryu admired how Teach, like Noland, refused to be bound by the World Government's rules. Moreover, he recognized the opportunity that joining the Blackbeard Pirates presented – a chance to challenge the system that he had come to detest.
Choosing to join Blackbeard was, for Shiryu, a decision rooted in his lineage, influenced by his time at Impel Down, and shaped by his desire to make a stand against the World Government. In joining forces with Teach, he saw a chance to honor his ancestor Noland, by becoming a part of a significant change in the world of One Piece. His story continues, interwoven with the legacies of the Montblanc family, the tales of Noland, and the path of the Blackbeard Pirates.