My theory is that Maribel doesn't become Yukari nor is she already her, I think that they live in parallel universes and are interconnected by their ability to see boundaries (and as Maribel's ability further evolves, manipulate them as well). Maribel might turn into a youkai but I believe that she will still be a human with Yukari's power and manage to consciously traverse the Hakurei barrier together with Renko and enter Gensoukyou.
For me, I don't believe that Maribel is Yukari or on progress of becoming like Yukari. Granted, while their abilities may be similar in nature, I like to believe that they are different people, since I don't really think that Maribel and Yukari are the same people, since Yukari is a lazy bum of a youkai. (Ironically, I haven't read any CD stories besides Dolls in Pseudo Paradise; just it seems unlikely that they are basically the same person or related.
)
>they look the same but at different age
>they have the same powers but Maribel just starts discovering them
>when asked about them ZUN made reference to Lafcadio Hearn, who changed his name to Koizumi Yakumo
>fans: "meh, could be a coincidence"
Like, c'mon, ZUN's reputation of a "trolling creator" is grossly overexaggerated. Theories are fun and all, but that just takes away mystery and adds nothing in return.
Taboo Japan Disentanglement is out.
Yup, once again, nothing happened.
I am sincerely sorry if voting "trolling" and its meaning were felt off in its worst light, since in this case I understood and converted "trolling" similarly to "good-natured and thought-out clues, insights, build-up(s), lead-push-on(s)"
There is quite likely some or enough facets of The Truth to all of you all quoted insightful statements written here, or otherwise implied or linked through.
As I have written in my apology earlier, by my humble opinion I have voted the 3rd "...not Related and ZUN(Jun'ya Ota)is..." ... even if it the last "Other..." has its own good merits , plus enlightening insights by you all by now , for an alt. vote.
My choosing of the 3rd option is motivated by obedience to canonical clues and proofs - I am sorry ,again, if these CAPS here are misunderstood ,but they are justly here for highlight-reading-aid especially thriugh longer text and details such as in the current One : " Sakuya Izayoi's BEYOND-HUMAN-LIKE/NON-HUMAN-LIKE/YOUKAI-LIKE ability/power/talent/capabilities" to ' Manipulate and Stop Time") .
(Clearly Hinted +with Quotes ) on the article of" PMiSS Perfect Memento in Strict Sense "just different ,only some times and It It Is well-feasible for' explainig and Insighting's coupled with quote reporting's sakes , different by mode of explanation and synthax like this :"...even though It is rare for humans (ref.+hint the prime example is Sakuya Izayoi ,but it hints to Reimu,Marisa,Marybel,the future-wise tò Sanae and Sumireko etc.as well)to hold and develop such powers/capabilities there are Majorly Particular one with one being the Heroic Human Legend in this Article of hers(ref.+hint of Sakuya and around similar Caliber and Potential AROUND and BEYOND-HUMAN-LIKE/NON-HUMAN-LIKE/YOUKAI-LIKE) for them it still possible to..."(ref.+hint that they are deemed HUMANS but also MOSTLY BEYOND-HUMAN-LIKE/NON-HUMAN-LIKE/YOUKAI-LIKE , but POINT-AND-BORDERS-OF-CONTACT between the 2 NATURES are questioned,theorized with also rumor-mentioning as well ; It Is mainly a case ref. with Sakuya Izayoi but not exclusively if expanded Upon Power/Ability-wise ) ..."
.My possible alternative vote for the Last Option is according due to the likely enough reasons I have just written between "(...)"especially the CAPS and sorrounding ones.
I am trying to explain and share that "being Truly Related Somehow" would "bear more justification and purpose, deeper and more founded " than the truthfully present and already pointed out by you all"Faithful Correspondences,yes,but more around 'on Border and Indirect-wise enough Similarities and Proximities" including "Talents ,Powers,Appearance that could very well mainly belong and involv the Mostly-Youkaifying-Like-type but still a Particular Human with Very Particular Powers relatively similar but not beyond ita relatively BOTH by their nature,kinds of abilities,developmentnor developed wholly complete nature as Yukari..."
Border Manipulation and Crossing by Yukari 1st and Marybel 2nd and partial) and Sakuya's Time a Manipulation and Stop are Canonically Clearly Hinted and Confirmed to be respectively a Particularly and Mysterious Youkai-like Youkai,and Outside-World-origins Denizen of Gensokyo,and Outside Worlder.