1.

an·ti·dote
ˈan(t)iˌdōt/
noun
1.
a medicine taken or given to counteract a particular poison.
synonyms: antitoxin, antiserum, antivenin
"the antidote to this poison"
something that counteracts or neutralizes an unpleasant feeling or a substance that cancels or opposes the effect of a remedy.

And to Niall, Harry was his perfect Antidote.