I disclaim all that follows.
All the team really knew about the guy was what he looked like and that he did not exactly treat Mary right. Well, that and that the boy touched much more than a thirteen year old should in a relationship.
Mind links did a lot for the Young Justice team. They allowed free uncensored communication during missions, during meetings, and most importantly when spying on people. Due to Superboy and Beast Boy's heightened senses, the whole team had a front row seat to Mary Marvel's not so great relationship with her boyfriend.
Team members would feed memories directly to one another as they spied on the young girl and her unapproved visitor. Like when she changed her uniform to one with a lower neck line, the boy teased her endlessly about trying to get men's attention nearly bringing Mary to tears. The next day, the couple acted as though the conversation had never taken place and went back to laughing and joking around.
Now, the team had conflicting emotions about the girl's relationship and had been on the verge of interfering for quite some time now. Superboy, naturally, just wanted to punch the kid's face in and call it a day. Miss Martian insisted they have a talk with Mary about her verbally abusive relationship with him. Robin and Kid Flash were hell bent on just breaking the pair up with some convoluted plan no one else really thought would work. Artemis wanted to scare the boy off and make sure Mary knew not to bring any more civilians to the base. Zatanna wanted to out Mary to Captain Marvel before the girl went too far. Beast Boy kept pin balling between everyone else's ideas so fast no one knew what to think of it. Rocket kept claiming that Mary had to have more sense than what she was showing by staying in the relationship.
Aqualad knew something was up though. Today he finally pointed out to the team something they never seemed to realize in the whole mess. Captain Marvel might be naïve and obtuse but nothing got past Batman. Intruders would be prosecuted as soon as the bat found out, he was sure. That meant that Batman must know the boy and from how Robin acted, his protégé did not. Right now, they were doing their job, as they often do in their spare time. They were performing covert operations for their own reasons.
Seeing through the eyes of a bird perched in a tree nearby the couple, the team had a perfect view of Mary Marvel trying to snuggle up to the distant boy. "C'mon Billy. Please? I am so tired of hiding our relationship from the team. They'll understand, I promise!" Mary had been wanting to tell the team of their relationship since she joined the team a few months ago. This Billy kid did not want the Young Justice to know who he was. What kind of thirteen year old did not want to meet superheroes? Billy Batson must be evil.
The dark haired boy turned to the girl and flicked her with a pout. "Mary, I was nice enough to let you come here. Don't even try it. I'll let them know when I'm ready." She scooted closer and her boyfriend scooted away irritably. It brought back memories of the boy's begrudging admittances of love the team heard. It should not take so much begging on a girl's part to hear that the boy she liked loved her.
"You know Billy, it's not good to lie." Mary Marvel elbows the boy who glares back in response. Mary sighs exaggeratedly. "What ever happened to being so excited about having me around? It's almost like you don't love me anymore." The girl pouted turning away to make her black curls bounce.
Billy gave her a look the team had seen many times before when spying on the two. The dark haired boy looked annoyed, tired, and not at all in love with the girl who was so enamored with him. "I am happy I found you Mary. It's just…" The boy stood and looked at the sky while spinning slowly his arms outstretched. "It's complicated."
"Don't you want to walk around the Young Justice base without them treating you like an outsider?" Mary Marvel did not know that the team would never let that happen. Billy was an outsider. He was horrible. They hated him.
Billy's blue eyes focused in on the girl's. "I am an outsider Mary. I cannot be on the team. I have my own team." That was it. Via mind link the tea decided this boy had to be evil. They had to tell Captain Marvel. They had to tell Batman. They had to put a stop to this.
Captain Marvel glanced nervously at Batman who just stared back in a strangely reassuring way. "I'll tell them today. I really will. I will." The tiniest twitch was like laughter from the dark man. Captain Marvel readjusted his cape nervously before stepping away from the Zeta tubes and making his trek to the living area of Mount Justice with the Veteran hero tagging along behind him. He walked in on Kid Flash playing video games with Beast Boy as the others just watched halfheartedly. The whole team was assembled just as Mary had promised. "Mary, I'm here." He announced his presence and his sister beamed up at him from her perch on the sofa. Ever since they found each other, she'd been like the sun to him. As annoying as having a sister could be at times, Mary really did brighten Billy's life.
Mary was practically bouncing now and the rest of the team was staring intensely at the league intruders. Robin stood up quickly. "Captain, there's something we need to tell you about Mary Marvel." Mary looked confused and Captain Marvel was no less so before the teenager continued. "Mary has a boyfriend that she has been seeing here on Mount Justice behind your back." Captain Marvel's jaw dropped. Mary had a boyfriend and didn't tell him? He gaped at the caped crusader to see a multitude of twitches at the older man struggled to keep composure.
"You have a boyfriend and you didn't tell me?" Captain Marvel boomed at his shocked sidekick.
Mary fumbled visibly for words as she flew to his side. "No, I don't I swear." She spared a brief glare for her teammates. "How could I possibly hide a boy from you? We're always together."
Kid Flash was next up to bat as he zipped over to Captain Marvel, pushing Mary Marvel out of the way as he did so. "We have reason to believe he is abusive as well if not entirely evil."
At this, the world ended in a huge bout of laughter. "Bwahahahahahahahaahahaha!" Everyone stared on in shock as the last thing they expected happened, Batman laughed. It was not a subtle little dark chuckle. It was a rib splitting guffaw.
Captain Marvel fumed at Mary Marvel. "My sister is dating a bad guy? Oh, who is he? I'll knock his lights out!" Mary merely spluttered protests to the soundtrack of uproarious bat laughter.
It was then that the team realized their error. Suddenly, reluctant admissions of love seemed just about right. Tugs at the hem of Mary Marvel's skirt were less flirtatious and more mocking. Flicks and light punches turned into siblings showing affection in ways only they can. In a moment, Billy turned from the worst boyfriend ever to the best brother ever.