What happens to my Helixes if I cancel my subscription? Locked

Any Helixes you bought through Helix packs are yours to keep and will not disappear when you cancel. Your monthly subscription Helixes stay usable through the end of the period you have already paid for, but once the subscription fully lapses you will need an active subscription again to spend Helixes. Note that premium chapters you have unlocked are also tied to having an active subscription: if your subscription lapses they lock again, but your unlock history is saved, so everything reopens as soon as you resubscribe.