What they had was simple.
Sneaking off together after missions to various hideouts in the Cave – he knew all the best ones, of course – to hide from prying eyes; simply talking about whatever was on their mind that day. He would console her about the circumstances of her joining the Team and she would listen to whatever he could to her about being Batman's partner. Their talks could span the whole afternoon or take up the few minutes of peace they had.
Eventually the talking stopped. Biology took over and as hard as they tried to fight it they eventually gave in. A brush of a kiss upon a burning cheek grew to more fevered uses of their mouths. Hands eventually travelled to uncharted lands, the emotions they drew forward the only evidence of their travels. And at age sixteen, after three years being in something undefined, they claimed each other's innocence and became official.
Then it became elaborate.
Being in an official relationship meant more ties to each other. Obligations they had to see through. The casualness of the past was gone and left room for something to grow. It was nourished by the days together, nights shared, and even the time separated. Something that bloomed under the affection each received and conveyed. Love.
It consumed them slowly; filling up holes left by others and shoving its way to the surface. It grew to the point of no evasion and of no trifling. It demanded to be said, demanded to be heard. It bubbled up inside until they felt like they were going to burst with it. Then one of them said it; in the smallest whisper that the other almost didn't catch. Unable to keep it down any longer the sentiment was returned. Curled up in each other they felt right, they felt unconditional.
Eventually it became complicated.
They couldn't control the way they felt about each other. Thoughts strayed on missions to the well beings of the other. Attention split between them and the job at hand. Plans constantly switched to accommodate themselves and ultimately risking everything. It became too dangerous to be together as everyone tried to tell them.
But they refused to listen. They blamed their failure on any other possible reason. Denial clouded their thoughts and their skills as heroes. What they thought provided them strength actually hindered their abilities. It wasn't until a grave mistake on their part almost cost them everything that they realized what was happening.
Then it became simple again.
She joined the Justice League and he stayed with the Team with his new alias. They had talked and mutually agreed that they had to end their relationship. They remained friends, close friends actually. They resumed their old ways of talking during their free time in the pockets of shadows splayed around. She would reveal the emotional distress it caused her to be working with her father, but not really working with her father. He would gripe about the freshmen and his responsibilities towards them, confiding to her his slight hesitation about leading them.
Just like the beginning they struggled to stay platonic. They were careful of what they say and how they say it, trying not to read into what the other said. But the love still lingered inside them, constricting around their hearts to the point where they had to stop and get some space. They were a few kisses, a couple of caresses, but those could never lead to anything. They happened to ease the loneliness they both felt between other partners. It could never evolve more than simple.