This will be long.
Losing a Grimmchild is extremely painful emotionally (from seeing their memories and attachment, which is not allowed) and physically because out of nowhere, they will suddenly feel everything that happened to the child through their flame connection (it is a Grimm's equivalent of a soul) this is very bad.
Losing The Nightmare Heart takes a toll on the entire Troupe but it's worse for Grimm as he was naturally connected to it, because of this Grimm becomes sick being physically cold and emotionally distant he becomes really angry with Brumm for telling Ghost to banish the Troupe and himself for letting his child die.
He starts needing a lot more sleep then a Nightmare Eater normally does, he wears the Grimmchild charm like a necklace after fixing it.
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Headcanons and Au Canons
FantasyI am bored I use bases and modify them to fit the design better the only thing I make is my idea and/or version of the character unless their an Oc of mine