You died doing what you loved most protecting him.

You where on the mission in the tunnels and instead of him getting shot you got shot and died, you of course stay by his side in ghost form tho it might scare the living daylights out of him because he can smell the cherry blossoms purfume that you always would wear.

He ends up finding this song and playing it while he's working out and pretending you are still beside him alive

Eternity by Alex Warren (Pull up the song on Spotify or whatever you listen to your music)