Natsume said she had only half the diary.
As such, the person on the motorcycle was
not Natsume, or somebody working under her or in cahoots with her.
So who was the person on the motorcycle who ripped half of Ringo's diary out of her hands?
I think that it was... Kanba.
And I think he still has it, and has it hidden away somewhere for safe keeping.
Here's why I think this is:
1. Kanba was clearly getting impatient with his brother's methods of acquiring Ringo's diary. Kanba explicitly said that Sho should just steal it already. This was said not long before Mr. Motorcycle showed up.
2. Being the proactive guy he is, Kanba decided to take things into his own hands (no pun intended
) by trailing Ringo and Sho back in Episode 9, patiently waiting for the opportune moment to strike, and take Ringo's diary away from her.
3. Once that moment presented itself, Kanba floored that motorcycle and went for the diary. Kanba wanted this to be a neat and clean operation, doing the least amount of physical damage to Ringo as possible, given that Ringo is his sister's BFF. That's why he didn't just try to beat Ringo up, knock her unconscious, and take the diary
that way.
4. Kanba was "white as a sheet" upon hearing about Sho's really dangerous accident, at least in part because his theft of half of Ringo's diary was the catalyst that lead to it.
5. Kanba told Ringo to hold on to her remaining half of the diary since he felt it was safer in her hands than his as he attempted to hunt down Sho's kidnapper in Episode 10. Basically, he didn't want Sho's kidnapper to have an opportunity to steal the remaining half of Ringo's diary from him.
If this is true, then it makes the conflict between Kanba and Natsume more interesting. They both have one half of the diary, which might also be one half of the Penguin Drum.