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I think Duke and Bao (Clownhunter) would be childhood friends.
Bao and Duke both grew up in the Narrows. I imagine Duke meet him at Bao's parents pho restaurant and dragged him off to play together.
The two would run through the steets, play tag with the other Narrows kids, do their homework, which at that age was mostly just colouring in, in a cornor seat at the restaurant.
When Zero City happened, Bao's family would move in to the apparentment block with Duke's family and others. It was that point when their relationship chnaged from friends to brothers.
They were attched to each other. Duke took it upon himself to be the older brother (dispite only being a year older). He would tell Bao to do riddle while he went out to get fish, no matter how much the other protested (or how much he was scolded when he got home). Bao use to pretend to be more scared than he really was so he could stay with Duke, hoping to protect him a bit.
So their bond grows, as they start middle school they don't hang out as much being in different grade but are always together at lunch. They get into petty fights, play video games together, were general hooligan(/aff) on the streets.
Then Bao's parents we killed by Joker. He started living with his aunt and uncle, Duke and his parents would bring food around, mostly as an excuse for Duke to just sit with him. In the grief, Bao closed himself off, stopped responding to Duke's messges, stopped going to school, stopped leaving the house.
Duke tried to keep contacting him, he would show up at his house, force Bao to sit with him and do his online school homwork while Duke did his, like when they were kids in the restaurant. That stopped by Joker got Duke's parents too.
Duke got washed away in the foster system, in being Robin, in becoming Lark and later Signal, in joining the Outsiders.
The Joker War started, Bao became Clownhunter, was washed away in his bew found duty to finally stop the Koker how Batman never could.
They almost naturally just stopped talking.
Until, while waiting for Jay to be able to take custody of him, Bruce sent him to Gotham Academy.
There was Bao again. The first thing they did was fight. They were both mad at each other for abandoneding the other. A few hours later they made up. Dispite not talking for so long, they were best friends, more than that, brothers.
They caught each other up if their lives, keeping the vigilante business out. They started messaging each other regularly. Duke would fight the people who builied Bao, the two spending a few afternoons in detention because of it. They would just sit with each other, happy to be in each others space.
Bao joined Batman incorporated which caused Clownhunter and The Signal to finally meet.
After taking down Professor Pyg's operation, some of the Batkids, lncluding Signal, arrived for clean up. Clowerhunter knew about the Signal, he was Gotham's hero. Signal knew about Clownhunter, he'd read the file Batman had on him. When they saw each other they immediately understood who was under the mask.
There wasn't a chance to talk that night, they didn't want to reveal each others identifies to the other vigilante's and Ghostmaker had an argument with Nightwing, quickly ending Batman Incorporated's time in Gotham.
They meet the next night, in costume, on the roof of their old apprentment building. Once they shared during Zero Year. Not much was said, Duke didn't mention the people Bao had killed, Bao didn't mention him joining the Batfamily. However, they understood each other mor now. Not much needed to be said. Duke gave him a Signal beacon, a promise to help if Bao ever needed. Bao gave the same sentiment. The two parted ways again.
They knew at that point they were closer than ever, dispite being so far apart. They had their own paths to follow but they felt more secure now knowing they had family in the hero community who will help them in any way.