I believe it's Shiryu's father.
Shiryu chose to become a pirate for a reason, and I always wondered why. He could easily make his own way in the world, with his strength. He doesn't need to serve under Teach.
Shiryu could have easily used the chaos of Luffy and Blackbeard to escape Impel Down. He CHOSE Teach. Why?
Well, Teach seems to want to truly destroy the Celestial Dragons. God Valley was Oda showing us this dude HATES these fuckers for what they did to his family.
So, I think Shiryu, in order to join Teach, has to have a very personal grudge against the WG, like Teach does.
Shiryu's father is a God's Knight that raped a Pirate in Impel Down. The child was born as a prisoner, but, due to their strength, was given the opportunity to become a "servant" of the World Government, and Impel Down. So, he rose up the ranks, hoping to free his mother, but, she died in the prison.
This is why Shiryu likes murdering the prisoners; it's his way of dealing with his grief.
Eventually, Shiryu realized, from Shiki, that Impel Down was bound to fail.
It wasn't some impervious hell.
It was a ticking time bomb, and someday, it would explode.
Shiryu wanted to DESTROY Impel Down. And for that... he needed a crew. The Marines would never do it. Shiryu had realized the Marines were worse than Pirates, on many levels.
No. Shiryu needed someone with a real plan, and real power.
Someone who hated the World Government and Celestial Dragons as much as he did.
That man was Marshall Davy Teach, Blackbeard.
Shiryu's Fruit, the Suke Suke no Mi, allows him to easily infiltrate a prison, while Blackbeard's Gura allows him to destroy the prison.
Blackbeard is going to learn to use the Abyss via his Awakening... and I think it might have already happened.
If Lulusia is Blackbeard's second home, after God Valley, Teach is about to GO NUTS.
Using the Abyss to enter Impel Down, Blackbeard will free the entire prison... and then send them up the Redline.
Shiryu is going to get revenge on Impel Down, the Celestial Dragons, and the World Government.