Mari-
She shouldn't have yelled at Chat. It wasn't his fault Adrien broke her heart. Yet, if she apologized to him, he might take it as a hint that she liked he back and that she had just been upset that someone she cared about broke her heart.
As she thought these things, she joined a website chat called Broken Heart. She instantly began chating with someone called NoirHeart. She was going as NetteLove.
Two weeks later, they decided to meet at her house so they could know who each other was. Neither intended to be romantic, so they wanted to know what their respective friend looked like. That morning at the agreed time, Mari stood on her balcony watching for NoirHeart. Suddenly she heard two feet hit the ground behind her.
"NetteLove? Your Marinette?"
"NoirHeart? Your Chat Noir! And how do you know my name?"
"I-uh must have heard it in the bakery." Chat said.
"I hear Ladybug is the one who broke your heart."
"Yeah, apparently she had her heart broken earlier that same day. I wish I knew who that jerk was!"
Marinette almost said 'Adrien is not a jerk!' but held her tongue, she wasn't going to let him know she was Ladybug.

Adrien-
(Chat)
Mari said, " You know that model Adrien? He's the one who broke MY heart."
Chat almost started. His rejection must have hurt her more than he thought if even have of what she said online was true. That was part of why they met up, to have a shoulder to lean on in the depths of their despair, which, alongwith emotional comfort, provided a defense against akuma attacks.
They got to talking about their everyday lives, Chat using generalities of course as he didn't know what would happen if he accidentally revealed himself after breaking her heart.
Soon they were better friends than Mari and Alya or Adrien and Nino, though Alya would burst into flames if she heard of it. Chat grew accustomed to visiting Mari's house everyday, rain or shine, and after every akuma attack.
(Adrien)
As their friendship grew, Adrien found and searched everything he could on Ladybug's identity so he could find the boy who broke her heart.
There were endless fake social media accounts, most of which he discarded for two reasons: 1. LB wouldn't tell anyone who she is and most accounts were people saying who they were and that they were Ladybug. 2. People gave incorrect accounts of akuma that had and hadn't happened. Those that did not outright discount themselves were three-
•LadybugIsHere
•IAmLB
And the most promising of all-
•MyNameIsBugaboo
No one bit LB knew he called her Bugaboo, no one. Which is why only her most devote devotees were friends/followers/subscribers to this account, probably thinking Bugaboo to be done code name.
He also had an anonymous subscription to the LadyBlog, and he laughed at failed attempts to find HIS identity that occasionally showed up.
At some point, Alya had taken pictures of their classmates and filtered them through the MiraculousApp to see if any of them were Paris's heros.
Glancing at them to see if it showed him, which would be a problem, Adrien stopped dead.