A young university student haunted by his past decides to return to his work as a hero to keep the city safe from a strange visitor from another time.

Hi Angelous vera ,

This is how I see it so far:

A young man decides to become a superhero and keep the city safe from a malevolent time traveler.

However, I think you have to clarify certain points in your logline:
A young university student (what is the significance of mentioning the lead character is a young university student?)
haunted by his past (What past? How did it affect the present?)
decides to return to his work (What is his work? and why “return”? Did he leave his job at some point?)
as a hero
to keep the city safe (How?)
from a strange visitor from another time who wants to do what?

I hope this helps
Good luck