I believe that they are various factors that lead to Quicksilver saving Costel. However I do not believe that the main factor was to impress the unnamed girl. Mainly because when facing moments like that your mind can't process what's going to happen until it actually does happen. Quicksilver saw that Hawkeye and the boy were in harms way and the only thing that was in his mind was saving Costel. But for a split second he could have seen himself in Costel. Quicksilver knew that this boy has a sister and Quicksilver most likely didn't want another Segovia girl growing up not only without her parents, like her sister, but without her brother. Yes the unnamed girl was a bit older but the feelings of losing someone is the same. Even though Quicksilver left his sister when he died, the end results were not that bad. A boy was reunited with his sister and Scarlett Witch is now an Avenger. Yes he may have left her in this new and weird world but he knows that she is strong and can overcome anything, even his death.