Like we already said - Marinette, Félix and Kagami were the only ones who knew what's going on with Adrien. Even though Félix and Kagami were already familiar with his household more, Marinette had a loving family and never had a better view on his situation but she still understood what's wrong, but it's not just that. It's all about her approach. Félix mocked Adrien "unlike you I can say no to your father" and Kagami yelled at him "who's pulling the strings of your life". Even though they didn't have the same intentions they both told him the same thing "you have to put a stop to it". The thing is that Adrien can't because Gabriel doesn't care about what he wants.

But Marinette, when she asked Adrien "why don't you tell him" and he told her that his father doesn't listen to him and doesn't care, she looked away and said "I understand you".

She didn't need to hear more from Adrien. She didn't tell Adrien to become braver, stronger, she didn't tell him "you're not trying hard enough" which people tell abuse victims all the time, she didn't need to hear more. She understood him. That's exactly why he told her "you're the only one". Because she really was the only one who really understood him.

That's exactly what abuse victims need. They need your support. They don't ask for anything from you, only compassion. Otherwise they feel guilty as if they are responsible for what's happening to them. That's all he needed from a friend. That's why he's holding her hand, that's why he's looking at her like that. She's his safe place. She understands him. In the crowd full of people who don't see him being unhappy and two who see it but they make him feel bad about being unhappy, she was the only one who was nice to him. He found out that he has to leave his home, imagine how he felt. He needed this more than anything in the world and she gave it to him.