>>713871615
>>713871754
The ThornRing is literally just a thorn from a rose. Asgore grows plenty of roses, he hands them out to Kris and Rudy like candy.
On the Normal Route this would allow you access to the Twisted Sword (and potentially be used as an appropriate endgame weapon for Noelle roughly on par with the Black Shard/Twisted Sword and Justice Axe, assuming that Noelle will rejoin your party) and on the Weird Route they could have Noelle symbolically "equip" it to Kris to consummate their relationship. It would also be thematically appropriate as a drop if the secret boss is a Flowey analogue as has been commonly speculated.
Conveniently, if Noelle's original ThornRing was taken to make the Twisted Sword for Kris, she is uniquely able to equip Kris's Black Shard as a backup weapon and still be an effective attacker. The Twisted Sword and Black Shard both have the highest ATK in the game (+16) and both have the "SlayDark" effect as of Chapter 4, which makes them extremely effective at killing Titans and other Dark-element enemies.