Dracula smiling and saying "Marie" when he looks into the Mirror of Fate after killing his son Trevor. Even as the unholy lord of darkness, Gabriel still remembers and loves his wife Marie.
Dracula, under the impression that he'd failed to save his son's life, personally gives him a tomb deep inside Castlevania.
Going further with that, as Simon is approaching the room containing Trevor's Combat Cross, you can see in the upper alcove to Simon's left that Trevor's clothes have been preserved.
When Simon demands to know who Alucard is after staking Dracula, Alucard simply cradles a hand on Simon's cheek lovingly like a parent would to their child, before reluctantly leaving him.
The secret ending can get the heart stirring. Simon says farewell to his father as he stands on a hill, watching the castle crumble and Alucard fly off in bat form. After being orphaned and seeking revenge for so long, it seems that Simon is at peace, having finally seen his father once again, despite how much he's changed.
to Simon's left that Trevor's clothes