Okay, this answered one question and created several more. So it's clear now that the girl is dead and is now a ghost attached to him, but there were some hints that definitely stuck out:
First, how can Yasu see her? Two possibilities, either Yasu herself is dead too or Chiaki let herself be seen.
Second, was Chiaki killed by MC? It's still to soon to tell but I believe that's not the case, according to the profiles at the end, Chiaki is 11 years old and her ghost self is 35, meaning she has been dead for 24 years, that'd mean MC was 7 years old when she died. There are other proofs supporting this, such as Chiaki referring Yasu as "Yasu-chan", an honorific used only for younger people; the mysterious woman calling her "Chiaki-onesan" ergo Older sister Chiaki; and that her memorial is in the highway, implying she died in an car accident.
Third, why is she attached to MC? We don't know yet, but we know it wasn't accidental, because Chiaki mentioned "she choose him" and that he will "take her to his finish line", so she definitely has a plan of sorts for him.