What should a committed and volunteering girl do to repent after she impersonated different personalities, including that of a boy, online, causing people to become attached to her, and she now feels distressed by the lies?

Lying in jest is a major sin (kabirah) and a specific warning has been mentioned regarding it. The Prophet, peace and blessings be upon him, said: "Woe to him who speaks to make people laugh, and he lies; woe to him, woe to him." And the liar must repent to Allah with a sincere repentance that fulfills the conditions of repentance, and what came to mind was from the whispers of Satan. Likewise, one must repent from conversing with young men except for a valid Shariah-compliant need and within the Shariah-compliant guidelines.