Another Billy from where?
DamiBilly are already dating in this world, only the Bats know Billy is Captain Marvel because he drops by regularly to have a date with Damian. But if it's for a mission, they are still relatively professional, as in no PDA until the mission's over.
Damian is 17, Billy is 15, Jason is 26.
One day, like usual, Billy is in his Captain Marvel form, the Bats are all gathering around for some magic-related mission in Gotham that needs the Captain's consult. Things got dealt with. Things are going well. It's time to go home.
Jason picks up his helmet, ready to go back to his safe house to take a nap and the Captain suddenly hovers towards him, his face is still annoyingly cute, and bright, and free of all the misery in this world.
Jason: What do you want, Thunder?
Captain Marvel: Going home? Aren't you going to drive me home?
Jason: What? Why are you talking to me? Go talk to Damian?
Captain Marvel, confused: Why would I talk to your brother?
Jason, even more confused: "Your brother"? What? Are you two fighting?
Captain Marvel: Is this another prank? Are you pranking me, Jason? That's not nice. Is this why you have been so cold to me today?
Jason:... Alright, this is weird as fuck. How have I been weird? What you want from me, spit it out.
Captain Marvel: You haven't kissed me at all today...
The whole room finally notices the little conversation on the side, and everything just stops. Everything. All the Robins turn their head towards those two, with horror on their face. Except Damian, whose face has skipped from shock to straight up murder.
Damian: What is the meaning of this? I demand an explanation, immediately.
Jason, disturbed: Billy, what- I- I have never- OUCH! DAMIAN! STOP THAT! INNOCENT TIL PROVEN GUILTY! INNOCENT TIL PROVEN GUILTY!
Captain Marvel: Damian stop that! Bad Damian! Stop attacking your brother!
Damian: Step aside, Billy. I can forgive you for falling for Todd's seduction, but I cannot forgive a man who would commit the most treacherous and unforgivable crime known to man! He dies tonight.
Captain Marvel: What? Why? What crime?
Damian: Seducing a happily married person.
Captain Marvel: I am so confused right now? What happened, even? I am happily married? To whom??? Damian, I am dating Jason! There's no one else.
Jason, screaming from behind the couch: NO! WE'RE NOT!
And by that time, all the Bats have figured out that it's probably an interdimensional problem, and this Billy is probably not their Billy. All except Damian, who is going insane from jealousy right now. But are they going to do anything about it? Nah, let's just tape funsies- for evidence, first.
Damian, vibrating from fury alone: Jason? You? Dating Jason? What kind of blasphemous claim is this? Billy, you're dating me! I am your boyfriend! I am.
Captain Marvel: WHAT? BUT YOU'RE, LIKE, A KID?
Damian, with the coldest voice no one has heard for a while: That's it. I'm killing Todd first. And then you and I shall die together. Father, honor my last wish and have us buried in one grave.
Yup, and that's where Batman finally finally decides to step in.
It turns out there has been a mixed up between Billys from different dimensions. Apparently, nothing else really changes much, so both of them don't realize it. The only major difference is the other dimension's Billy is a little older than Jason, instead of being the youngest here. Those two have known each other since Jason's Robin days, and they have been dating after Jason decided to turn his life around for the better.
You can imagine the same situation happening in the other dimension where Billy tried to kiss Damian, as Captain Marvel, and Jason almost blew up the Batcave with everyone else inside in his moment of weakness.
When the Billys return to their dimension, they are faced with their extremely grumpy boyfriend.
The other dimension:
Jason: So you are dating Damian in another timeline.
Billy: Not me. The other Billy.
Jason: Yea, yea, sure. Hmm. I've never knew you two would be a match. Hmmm.
Billy: Jason, please, I literally see him as a little brother! And my co-workers' son! He was barely a pea when I first met him!
Jason: Hmm, sure. He's almost 18. Next year, to be exact. I think he's going to be tall. Maybe taller than me. Do you like tall guys? You liked me because of it, right? Damian's going to be tall. Hmm, should I go shoot him in the knees?
Billy: Jason stop being paranoid, would you? When I started liking you, you were literally smaller than me. And ugly. Too. You kicked me in the knees and stole my bread!
Jason: But still.
Our main world.
Damian: I'm going to strangle Todd in his sleep.
Billy: Damian, no.
The whole family gets a good 3 weeks of jokes and laughter, and shit and giggles, except Jason. UNTIL another mix-up happen, and of course, the same situation happens again. The only difference this time is the other dimension's Billy is dating Tim.
And then, something clicks in those geniuses' head.
There's a pattern there.
Billy always ends up dating a Robin of his age.
And they don't even know what is more disturbing:
They all having the same type, and that type is Billy Batson, either as the powerful Captain Marvel, or the stubborn but kind orphan from Fawcett.
Or the a paranoid Damian with a sword randomly appearing in their room at night.
