RPG Insanity

One of the rewards for subquest 18 in New Game+ is Tenebrant Lily given to Lucile by Prince Laird.
Is there any use for it? I've been talking to every NPC but I can't seem to find any use for it.
Is it useless like the Legendary Medal or is it something extra like the Weed Supplement?

I honestly don't remember. However, I checked a save file (game cleared), and the Tenebrant Lily is still in my Key Items. So, you're probably correct that it is pretty useless. Maybe just a "romantic" token?