I've seen the headcanon that Leo or Raph as Casey Junior's Dad (Adoptive dad ) and I like it.
But Imagine if it was biological.
Don't think about the logistics just think about how funny it is.
Because the idea that they discover it in the dumbest way. Like they assume he called (which ever turtle) Dad because they assumed it was a habitual accident from the future or he was adopted by them.
Like he doesn't have any obvious turtle features up front but he likes water a little too much to be a normal human but they assume it's because water was scarce back in his future.
Rats would've been part of his diet regardless of the limited food. (No Donnie not sulking about having to temporarily share his Pinky stash)
Maybe he has that weird killer instinct they have (they were genetically engineered to be soldiers) but once again it deduced he got the trait from living in a literal Apocalypse timeline.
(I like to think depending on the turtle dad Casey would have a different trait. If it's Raph, Casey has a weird tongue, if it's Leo, Casey has glowing stripes that is invisible in most lighting (think blacklight), if it's Mikey, Casey has sunset spots (easily mistaken for scars) and if it's Donnie, Casey has softshell spots and ridges of his back.)
When they finally figure it out it's like: "Why didn't you tell us?" "You didn't ask"
Imagine the reveal was because he can do the eye thing. Like when he's really focused, distracted or angry and they notice and are just like what?
"Wait, humans don't have a third eyelid" "They don't?" "Maybe they evolved in the future" "But can't April do that too?" "That's the reflection of her glasses" "Does Cas- "No Casey does not" "Maybe we should just ask"
"Oh? I got it from my Dad" *Still don't who know that is*
I can’t think right now the brain fog came back there’s fog horns blowing in my mind right now lighthouses are flashing behind my eyelids I cannot even PROCESS what you are saying your voice has been taken by the fog monster