This is a Katekyo Hitman Reborn! fanfic about Yamamoto Takeshi!! The heroine is an original character, not someone who transmigrated!!
Here's the blurb.
When a sword has been used for a long time and imbued with someone's feelings, it can give rise to a spirit. Existing somewhere between presence and absence, it hides within the sword and is known as a sword spirit.
Sword spirits are peculiar beings. Only the sword's owner can see one, and only if the sword spirit acknowledges them as its owner. Otherwise, no matter what, they cannot see it.
In this vast world, there are countless sword spirits, and naturally, many love stories have sprung up between sword spirits and their Hosts. Most, however, end in sorrow. A sword spirit is only a spirit: it cannot be touched, yet it can touch others; it cannot be seen by others, yet it can see them. Though they have feelings, the lovers can never be together...
I'm Xiao Ling. I'm really no good at writing blurbs, so this will have to do... I'm working on two stories at once, so updates may be a little slow. Please forgive me.