How many? Just, how many?
The book only stipulated four keys. This is the last one. Probably the princess key, considering the design is far more ornate than the rest.
I was pretty certain she was going to be the last one when she showed up and started talking about things from way back in Raku's childhood, close enough that she'd potentially be there for the events that transpired around the picture book and locket.
This got reinforced for me when Marika had that mortified face upon seeing her, and her reactions to Yui talking to her. She's terrified that Yui can unravel all of her machinations relying on Raku and the other's tenuous memory of that time that she remembers so much better, but refuses to elaborate on anything besides her time alone with Raku.
Yui kind of giving me somewhat similar vibes to Marika, but with far more assurance that her position with Raku is much closer from the get-go, being a childhood friend he actually remembers well because they spent several years together, not just one summer (it was a summer, right?). She doesn't have to jump onto and cling to him in desperation to convince him that they have a connection. She can just be herself, instead of some fabricated identity. She doesn't have to wear a mask and deceive him.
She is almost everything Marika wishes she could be for Raku, and it terrifies her that she's here now.
She is a pure, wholesome version of Marika, essentially. Where Marika is desperate, deranged, and obsessed, Yui is comfortable, caring, and genuine.
I absolutely despise Marika for being the way she is, but Yui... I get it. I understand. She is actually legitimately admirable. The distinction is the fact she doesn't have to resort to disguising her true self and deceiving Raku like Marika does. Any love built on a lie is doomed to fail. But a genuine love can thrive.
But it's clear Raku doesn't have the same feelings for her, either. Her love may come from a more genuine expression than Marika's, but she's just as doomed to fail. It's still only Chitoge and Kosaki that have a chance with him.
...
Also, just commenting on the chapter, but I did not see Shu knowing her, too, but I feel like a big dum for not realizing if Raku could remember her from his elementary school days, then she must have known Shu, as well, from the same time because of the boys' friendship.
She really does probably know everything. But I get the feeling she has no intention of sharing it anytime soon.