What is the ruling of Islamic law regarding a love story that ended with the death of the girl, and will the young man meet her in Paradise, and how can he seek forgiveness for her, especially since he is married and has children?

You must repent and conceal what happened. There is no harm in giving charity on her behalf and seeking forgiveness for her (O Allah, forgive her and have mercy on her), for perhaps Allah will accept that. As for her being your wife in Paradise, Allah knows best, but if she was not married in this world, then you can supplicate for her to be your wife in Paradise.