Just idle speculation:
It seems like Kumadori and Fukorou are a perfect yin-yang pair.
Both are physically very large compared to the other CP9.
Their douriki are almost equal.
Kumadori is a glutton for emotional pain, but Fukorou allows himself to be physically battered.
Fukorou lacks any self-control (see the gossip) but Kumadori has perfect control of everything (Life Return)
Kumadori claims to be hundreds of years old, but in the anime, Fukorou's voice sounds like a screaming infant (IMO).
I wish we could have seen more of their interplay… might have shown some interesting relationship.