How is this and falling merman connected?

I think their kid is the MC of falling merman, the love I treat of the merman.

I think their kid is the MC of falling merman, the love interest of the merman.

no...their kid is a side couple with his childhood friend, while the MC is the mermaid with the cat ML who they adopted (maybe???) from the lab which they hunt down...

the ML in the falling merman (which is the cat like person) is a test subject and he also calls the trash ML in this manhwa uncle.

The MC is kinda adopted,