GBP

Svidrigailov might be considered to be a romantic hero because of 3 things: he showed up at the right time, he wants to save Dunya from Luzhin, and he has money.
Svidrigailov came to Raskolnikov just before the relationship between Dunya and Luzhin ended. Which was heroic because now Dunya has a chance of a life without poverty even though Luzhin is gone.
Svidrigailov told Raskolnikov that he couldn't stand to see dunya with luzhin, and would marry her even though he doesn't love her. So, he is trying to save Dunya from a man he knows wouldn't be good for her.
Svidrigailov, also, has a lot of money that he is going to offer to Dunya (and she and the family need it) as well as money that was willed to her by his wife.
Svidrigailov is, basically, looking out for Dunya's best interests and financial support. He wants to be the one to help her, which in most cases would consider him heroic.