While climbing up the ladder that leads to OTHERWORLD during the PROLOGUE, KEL notices that something fell out of his pocket, only to realize that HECTOR is missing. OMORI and friends then decide to look for him for the time being.
HECTOR can be found in the top right corner of the first floor of DINO's DIG, which is located within ORANGE OASIS. You can access ORANGE OASIS only after completing the CAPTAIN OF THE SPACE PIRATES quest and getting the TRAIN PASS to travel there.
Upon digging up the patch from there, this will lead to a short cutscene of KEL reuniting with HECTOR. It is revealed that HECTOR now has a family and will not join KEL again on his adventure.
MARI (Hector (Quest)) Incomplete
MARI (Hector (Quest)) Complete