Is a girl sinful if she engaged in a virtual relationship then denied it, and the young man demanded retribution (qisas)? And must she seek his forgiveness after her repentance?

Your regret over your relationship with this young man and your breaking it off is a sign of your repentance, and turning to Allah in repentance is a good thing. You have the right to deny it if the young man exposes this relationship, because a Muslim who has repented should conceal their own past. Lying is permissible if it is the only means to achieve a righteous objective, though using equivocation (Tawriya) is better. You have not wronged him such that he can claim retribution (Qisas), and you are not obliged to seek his forgiveness. Rather, turn away from him and ignore him, and ask Allah for pardon and well-being and to protect you from his evil. And you should recite the Prophetic supplication when in fear: "O Allah, we place You against their throats, and we seek refuge in You from their evils."