Lyric discussion by goodintheory 

I have a completely different interpretation. There probably is a romantic backstory to the song, but I think the song is mainly about how someone that the Shin's guy really cares about is cutting themself. "Kissing the Lipless" refers to the fact that instead of seeking comfort from other people, the person finds solace in inflicting physical harm, the lipless being the blade.
It's apparent in several lines:

..."But you've got too much to wear on your sleeves It has too much to do with me" The person is covering up their arms to cover their scars

"The grey remains of a friendship scarred"

"You tested your metal Of doe's skin and petals While kissing the lipless Who bleed all the sweetness away"... The metal explicitly refers to the blade, which is euphemized with "doe's skin and petals" to refer to that fact the the person doesn't realize how destructive their habit is.

I found that the diction and metaphors strongly point towards the meaning of someone cutting themselves.

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