Porter Robinson

My guess is that the narrator is either supposed to represent a friend, family member or a soldier, etc.

The begining of the lyrics is sort of like an exceptance of the narrator's fate, and thanking someone for the life they've known, or the adventures they've had with them -

"Thank you, I'll say goodbye soon"

Telling them their eventual death isn't their fault -

"Though it's the end of the world,
don't blame yourself now."

And the classic 'I will always be with you' thing, though I admit I'm not sure about the part that fallows -

"I will surround you
and give life to a world that's our own."

And as it goes on "I'll say goodbye soon" turns into -

"I'll say goodbye now"

And near the end it all gets dissoriented and muddled as the narrator slowly fades away and dies

This song is about a group of rebels fighting back from tyranny. It is vague in what situation or area on purpose so you, the listener, can apply it to anywhere else. Specifically, "to be the lionhearted" is to be brave and not care about death, only your cause, not very different from the initial definition of the word.