FromSoftware certainly has a history of turning supposedly friendly characters into bosses. Gideon Ofnir, the Demon of Hatred (IYKYK), Gehrman, just to name a few. However, not every ally will turn their back on you. The giant made of rope in Sekiro: Shadows Die Twice is a very nice fellow, for example.
But, according to a recent datamine from YouTuber Zullie the Witch (via PCGamer), it appears that the rope man could have had more than just a momentary appearance. Helping you up to the Fountainhead Palace, he's only a small part of the adventure in Sekiro: Shadows Die Twice.
According to the files hidden within the game, there is unused AI script for Mr. Rope, also known as Wara ningyō, implying active gameplay was intended for it, rather than a simple cutscene. There are multiple theories about how the giant rope creature could have had gameplay associated with it. Some immediately think he could have been a boss, but as he's more than double the size of the Divine Dragon, it becomes difficult to imagine how we'd fight the thing with our flimsy sword.
Perhaps the more convincing theory is that we wouldn't have directly fought him, but would have instead had a section of gameplay where we fought regular enemies while climbing up the rope towards Fountainhead Palace. Alas, neither were to make it into the game, and all we got was a short cutscene.