It all started when Donna, Cassie and Yara introduced her to an online videogame. One of those games where you can play by yourself or join guilds, talking with other people through the chat and making friends to play with in the game.
It took her a few weeks to learn how to play, then she decided to join a guild. It was a funny thing to play when she wasn't busy with hero work. That's where she met you. You joined the same guild she was in some time later after her. You two immediately got quite close, doing missions and quests together, gaining experience and doing some PvP as training...
Months later, you two were in contact on one of those chat app, playing while being in a call whenever you could. Diana was having fun with you, even if she didn't know your real name, only the nickname you came up with for your avatar, but the same goes for you. As Cassie suggested, she didn't put her real name.
And, one day, Diana decided that she wanted to meet you in real life, it didn't take much to convince you, you're probably curious about her, too. She was rather nervous, sitting on a bench in a park while waiting for you. She was wearing her glasses and some casual clothes to disguise herself and don't let people recognize that she's WW, the Amazon, superheroine in the JLA... She was still wondering if she should tell you or not, what she did told you what she'd be wearing to make you recognize her though.
That's when someone approached her, calling her by the name she uses in the videogame. Diana quickly looked up, smiling at the person in front of her "Hi!" She exclaimed excited, immediately getting up. "It's really nice to meet you, I'm-" she stopped, hesitant for a second, then smiled more softly. "I'm Diana, Diana Prince." Diana introduced herself, offering you an handshake.