Without what character wouldn't Harry have been able to defeat Voldemort?
Well, let’s see.
Without Dumbledore, Harry would have zero clues about what to do.
Without Snape, Dumbledore couldn’t have communicated some of that info to Harry.
Without Lily, Harry would have been completely dead and Snape wouldn’t have turned to the good side.
Without James, Harry would never have aspired to courage.
Without Hagrid, Harry wouldn’t have known he was a wizard and wouldn’t have been in Gryffindor.
Without Ron, Harry also would have died and no one would have been able to get the basilisk fangs.
Without Hermione, Harry wouldn’t have known all the things she taught him throughout the series.
Without Neville, Nagini stays alive.
Without Luna, Harry couldn’t have found the diadem.
Without Ginny, the DA may never have been restarted, and the fake sword wouldn’t have needed to be sent to Bellatrix’s vault, revealing the Horcrux's location.
Without Dobby, Harry wouldn’t have escaped Malfoy Manor.
Without Kreacher, Harry wouldn’t have found the locket.
Without, Lupin, Harry would have been kissed by a dementor.
Without McGonagall, Flitwick, and Sprout, Hogwarts wouldn’t have been adequately defended.
Without Slughorn, Harry wouldn’t have known how many Horcruxes to look for.
Without Sirius, Harry wouldn’t have gotten Kreacher.
Without Peter Pettigrew, Harry wouldn’t have left the Malfoy manor.
Without Griphook, Harry couldn’t have got into Gringotts.
Without Molly, Harry would have lacked the loving nurture he needed to become the person he became.
Without Trelawney, Harry would never have understood his destiny.
Without the Seven Potters, Harry may have died.
Without Moody, the Order would have been left in chaos after Dumbledore.
Without Scrimgeour, Harry would have been found sooner.
Without Lee, Kingsley, Remus, Fred, and Xenophilius, people would have given up.
Without all of these and many, many more, Voldemort would never have fallen.
Harry probably played the most important role, but it was a group effort. Except for Umbridge.