since it was well received and I have more to add...more Splinter thoughts! a special thank you to @turtlecleric for inspiring a few of these thoughts.
The second Splinter realizes that you and one of his sons are falling for each other, he is quietly celebrating and secretly encouraging it. It's small things, so small that the turtle in question doesn't pick up on it.
"My son, accompany her! The city is a dangerous place at night."
It's started raining and it's awfully late by the time you get there?
"Stay there and return in the morning. I would not wish for you to take ill."
The two of you are always being sent out on errands, any little thing that means you will be alone together (and it will be the smallest, most time consuming thing but you're both so happy about it because it means hanging out alone), and you very quickly come to one conclusion: Splinter knows. And here you were thinking that your feelings for the turtle were a well kept secret, he must have known since day one (of course he did! The man's a master ninja and a single dad of four, nothing gets passed him).
"And you're okay with that?"
"I am. You make my son happy and Ive ever wanted is for my boys to be happy. And I know he will treat you with the affection and respect you deserve, Y/N."
The fact that you're Splinter's friend is a part of what's so endearing about you in the beginning. Because his dad likes you and spends time with you, so you're around a lot and he gets to know you and spend time with you even more, though the turtles rarely intrude on your time together.
But sometimes, he does join you when you're watching your shows together.
No, he's not really watching and, no, he totally doesn't like it...except that he does, he's so into it and you and Splinter are exchanging looks and trying to get him to watch with you. As a bonus, here's a few shows for each turtle!
Leonardo: Grey's Anatomy, Days of Our Lives, and Generations.
Michelangelo: Dark Shadows, Passions, Loving and The City.
Donatello: As The World Turns, One Life to Live, and Santa Barbara.
Raphael: The Edge of Night, The Young and the Restless, and Ryan's Hope.
It's the offer of sitting next to you that draws him in every. Single. Time. And Splinter's hiding a smile, pretending hot to notice that you're holding hands beneath the blanket and you're pretending not to notice that he's noticed and the turtle is STILL oblivious to it all.
Until he decides to talk to his dad and tell him how he feels about you, wanting advice on what move to make, if to make one at all.
"Why do you hesitate, my son?"
"I just...I just don't know if I'm what she wants. The life I can give her is so different from the sort of life most humans expect, what if it's never enough?"
"What she wants is to be loved. Do you love her?"
And when he decides that he is making a move, Splinter announces how happy he is after all, if it hadn't worked out, then all of his hard work would have been for nought, the turtle finally figuring out his father's game.
Because now you're together and Splinter might just be happier about it then the two of you, just by a little bit. Maybe it's the romantic in him but he loves knowing that two people who are so special to him have found happiness together.
β You can make sushi and he loves it, so having sushi together becomes a thing while watching your soaps.
β Once, you mentioned that some Purple Dragons had started hanging around your neighborhood and bothering folks. So one night, Splinter just waits for them in the darkness, follows a couple of dragons back to wherever the base is and opens a can of whoop ass on 'em, sends them all running far, far away. Then he just casually walks into your place, sits on the coach to order take out and watch TV, and casually announces that the Purple Dragons won't be around anymore.
"I gave them the guidance they've lacked all their lives. Hopefully, the lesson will stick!"
"I can always give them another lesson."
β He's very gentle with you and his son's relationship. wanting to make certain that you don't feel pressured or rushed in any way. And you don't. You're happy in your relationship with the turtle and your friendship with Splinter, something he's pleased to hear.
β Your watch parties for season starts/ending, holiday specials, or whatever are always themed. You make food and drinks that fits the show, set up the whole living room with fairy lights, make a bingo card for the show's tropes, and everything. And there's always "The Bet". You and Splinter make a big bet on something for the show and, somehow, he always wins, it drives you crazy!