Did you notice how Blackbeard’s teeth change whenever he fights, confronts, or even talks about someone with the Will of D?
For example, during his fight with Law, he’s missing two upper teeth and one lower tooth. The same thing happens when he fights Ace—two teeth missing on top, one on the bottom. Another moment to notice is his first meeting with Luffy (this happens only in the manga). Interestingly, he even “loses” a lower tooth when he’s talking to Nami, saying,
“You’ve got some balls for a girl!”
But as soon as he shifts focus back to Luffy, the tooth reappears.
However, when Blackbeard fights others, like Rayleigh or Whitebeard, his tooth pattern is different. In these cases, he’s always missing two teeth on both the upper and lower jaws. There are various theories about his shifting teeth—maybe you’ve heard of one.
Conclusion
It seems like Blackbeard’s teeth may indicate when he’s facing someone with the Will of D. If that’s true, then this could suggest Kizaru also carries the Will of D, based on how Blackbeard’s teeth behave around him.