The most recent Steven Universe bomb contained a lot of hints about the overarching plot, from Pearl's backstory to Lapis Lazuli's appearance in future episodes. There are many mysteries in Steven Universe, with clues hiding in plain sight all the time. And according to this new theory, it seems as though the fourth Pearl has been hiding under our noses for a while...
The Diamond Pearls
Before we get into the nitty gritty of this theory, let's establish some context. The Pearls are one of the most interesting of Gem classes: Peridot revealed to Steven that the Pearls are little more than servants on Homeworld...
"She's a made to order servant, just like all the other hundreds of Pearls being flaunted around back on Homeworld."
This explains a lot about our own Pearl, who clearly struggles with her self worth.
Pearl values herself on how well she can protect Rose Quartz and Steven, and her own backstory is delightfully mysterious. It hasn't been confirmed who she originally belonged to, though we have a theory.
All the Diamonds we've seen so far (Blue and Yellow) have been accompanied by a Pearl who matches their color scheme. This has lead some fans to speculate that our Pearl belonged to White Diamond, as her skin tone is very pale.
If this is true, there is still one Diamond and one Diamond Pearl unaccounted for... but have we already seen Pink Diamond's Pearl?
Hypothesis: Lion is Pink Pearl
This might seem a bit far fetched, but the theory by lennat actually makes a lot of sense. Lion has been a mystery since his first entrance into the show. Discovered by Steven, the Crystal Gems had no idea who or what Lion was, but it was later revealed that Lion had a strong connection to Rose. Lion lead Steven and Connie to Rose's secret cave, which Pearl found difficult to accept.
Lion also exists as a sort of "living bag", hiding many of Rose's possessions in his pocket dimension. And this might be the crucial hint we've been waiting for. As lennat points out in this post, while all the Gems use their gemstones to summon their physical weapons, only Pearl has been shown to store other items in her stone.
This hearkens back to Peridot's statement that Pearls "hold your things for you" - perhaps Peridot was actually referring to this gemstone storage ability. So, back to Lion. It's possible that Lion actually is Pink Pearl, as lennat theorizes...
"Who, and where, is Pink Pearl? Patterns thus far would indicate that she would have a pink color scheme and be able to hide things within herself. She also would have had a very obvious affiliation with Rose Quartz, who either is Pink Diamond or belonged to her court. Incidentally, one character fits that description perfectly: Lion."
This actually makes a lot of sense. After all, gems can change their physical form to become something other than humanoid. This would also explain why our Pearl didn't know of Lion's existence.
It's possible that Lion was meant to be somewhat undercover, a safeguard to protect the things (and people) Rose Quartz valued most. Or maybe Pink Pearl appeared as Lion to protect her own identity after she betrayed Pink Diamond. The plot possibilities are exciting, and even if this theory doesn't turn out to be true, it definitely makes for an interesting plot twist!