Apocalypse friend
noun - friend who, unlike the antonym fair weather friend, is there through all the fire and brimstone moments in life. When a person is unwell, heartbroken, jobless, homeless or even had a day from hell this friend is patiently there through the worst of it. They are the friend to the end.
Following a bad break up and the death of a well loved grandparent, I battled with depression. My apocalypse friend wasn't phased by any of the hell fire or brimstone, they held my hand and led me through the worst of it.