Gender transformation and a partial marriage plot, which you can ignore, of course. There's almost no romance, and the protagonist's point of view feels a bit like that of a doting mother. (The protagonist can evolve infinitely, but won't have nonsensical, power-breaking evolutions.)
Fei Wu transmigrates as a demonic sword and is given by a dragon to Luffy as a keepsake. Who knew that an accident would bind it to Luffy, forcing it to stay by his side?
Though it has a million reasons to refuse, when fate comes knocking, all it can do is help the free-spirited boy become king.
