So there was another person in the cemetary besides Risa? Possibly there to visit Aika too? My guess is Koharu....
Go read chapter 4 again.
Somehow people tend to miss this short scene in chapter 4.

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Chapter 9 - The Arrival
We are in a plane from Europe to Japan. Most passengers try to sleep to let the time fly by like nothing. One woman is coming back from the bathroom. On her way back to her seat she notices a girl listening to music. That's nothing special, but somehow she thinks that she knows her. First she blames her lack of sleep and continues to walk to her seat, but then she suddenly stops.
~Is this?~ ... She walks back to the girl.
~Excuse me?~
"Yea?"
~You, you are Eri!~
"Huh? And who are you?" ... The girl, Eri, looks up to the woman.
"Nacchi!"
-ssssssssshhhh!- ... makes another passenger
"Nacchi?" ... Eri whispers now.
~Hi..~ ... Natsumi smiles.
"What are you doing here?"
~Is the seat next to you free?~
"What? Ah! Yes!"
~You are going back to Japan?~ ... Natsumi asks while sitting down next to her.
"Well, we are both in the same plane, aren't we?" ... Eri laughs.
-noisy...- ... A passenger mumbles.
~I heard you are successful in France?!~
"It's okay."
"Didn't you even make 1st place in the charts of Germany?"
~Yes, but that was a while ago.~ ... Natsumi still smiles.
~So, why are you going back to Japan?~
~Meeting the family?~
"That's pretty much the reason, yes."
"And yourself?"
~Same, meeting friends and family.~
The conversations continues and from time to time another passenger complains about their chuckling and other noisy sounds they make. About one hour later the plane successfully lands in Tokyo. The two girls walk around the Haneda airport and see a billboard of "Wasuru Na Kusa."
~Look!~
"Junjun is cute, isn't she?"
~This reminds me, I should visit Yuko as long as I am still in Tokyo.~ ... Natsumi picks out her phone.
"Do you think she is still up?"
~I know that she still is up.~ ... Natsumi calls Yuko.
-Hello? NACCHI?-
~Yes, it's me.~ ... Natsumi laughs.
~Good to hear you.~
-Hum...yes, same here.-
-Are you calling from Europe?"
~What? No, I am in Tokyo.~
-You are WHAT?- ... Yuko can't believe it.
~Want to meet?~
-NOW?-
~Yes~
-Sure...-
~Wait a second.~ .. Natsumi turns to Eri.
~Want to come, too?~ ... She asks.
"Me?"
"Well, I guess I still have a lot of time."
~Great!~
~Yuko?~
-Yes?-
~Guess who is with me.~
-Who?-
~Kamei-chan.~
-ERI?-
~Yep.~
-I am speechless.-
~Where can we meet?~ ... Natsumi laughs.
-You are coming with the train?-
~Yes.~
-Then let's meet at Tokyo Station, I am not far away from it.-
~Okay, I will phone you when we are there.~
-Okay-
~See you later~
-Bye-
~Let's get the next train.~ ... Natsumi turns to Eri.
"Okay." ... Eri smiles.