To be clear, "The Demon" is what I call the version of Chara that exists within Frisk, the same one who talks to us at the end of geno and is the narrator of the game. That version of Chara is what I claim to be the soulless Essance of Chara, which attached itself to Frisk and was brought to life using their Determination.

To recap, when Asriel carried Chara's body to the surface and laid them down in the bed of golden flowers those flower seeds clung to both of their bodies. Alphys tells us that the flowers are a pain to work with due to the sticky seeds which refuse to let go, so if Alphys couldn't avoid getting covered with them even while only working with a few of them in a controlled laboratory environment, you can bet that laying in the bed of flowers made both Chara and Asriel absolute coated in them. Once returning to the Underground Asriel collapsed, spreading both his dust and the flower seeds over the throne room. These seeds then grew, with his dust likely working as a type of fertilizer as they did. This dust was thus absorbed into the flowers, it wasn't just sprinkled ontop like people say, Asriel's essance is literally is flowing through the flowers. Alphys then took one of these flowers and injected it with Determination, awakening the Essance and letting "Asriel" take control of the flower.

With that in mind, what happened to Chara? Well, their body was taken by Toriel and buried in the Ruins. It was still covered in flower seeds, so when they were buried so were the seeds. Those seeds then grew, using Chara's decomposing body as a fertilizer as they became a full bed of flowers. Sound familiar? By all means these flowers should contain Chara's essance just like the ones from the throne room contain Asriel's. If you took one of the flowers from the ruins and injected it with DT it would turn into a Chara version of Flowey. Following me so far?

So, what happens when we land on the bed of flowers? With how sticky the seeds are, they'd coat Frisk head to toe. The same seeds that as previously discussed, have Chara's essance flowing through them. So, Frisk is covered head to toe in Chara's essance, and as we know to create a being like Flowey all you need is Essance + Determination. And what is the single most concentrated source of Determination we know of? Frisk. Frisk had the Determination to overwrite Flowey's, they have the DT to refuse death, to even wrestle against the DT of God Form Asriel who says that he wont have full control over the timeline until Frisk gives up. Asriel, the being with 6 human souls and the soul of every monster in the Underground, does not have the DT to overwhelm Frisk.

TLDR: To create a soulless echo of a dead person you need that person's Essance, some DT, and some kind of host for them to inhabit. Frisk matches all 3 requirements, however while a flower has no will of its own and thus Asriel can take control, Frisk is a person with their own soul and their own mind, so this resurrected echo of Chara is only able to manifest as a voice in the back of their head.