Got into this thrilling series not too long ago, and this is my theory for the Clover mastermind.
First, after much assessment it won't be an alternate future Yotsuha or Uichi, tempting as it is. This is because of the mechanism behind this series' time travel: it's the mind, the consciousness that travels back, not the body.
Yotsuha loops back by reversing the state of the world, including her body, and only her consciousness and the watch persist continually. Thus were a distant future Yotsuha to travel back to her teenage years, she'd have already overwritten our Yotsuha to begin with.
So as long as we don't introduce some physical travel time machine (which I heavily doubt, and would be incredibly lame by this point), two Yotsuha's cannot physically exist at once. And let's not get into a future person possessing someone else's body.
By the same logic, it rules out an alternate future Uichi as well. Mastermind has to be someone else.
So who's my candidate for Clover? I'm going with the theory that it's Fuu-chan.
But not present-day Fuu-chan, the Fuu-chan whom Yotsuha knows.
A Fuu-chan from the original future who miraculously and unwittingly travelled back in time into her 5th grade self (6 years from the present day)
Motive
Let's start with the motive: Clover's goal is not simply kill Uichi, but specifically for Uichi to kill himself. Because per Ch 36, he apparently "killed" someone - in quote marks I state, because characters in this story may deem indirect causes contributing to someone's death as "killing".
We've also seen that Clover wants Yotsuha disarmed but not harmed at all. Or at least physically unharmed, they're perfectly fine with Yotsuha painfully losing all hope in Uichi.
So my theory is this: in the actual original timeline (ie without Clover's meddling), Uichi messed up somehow and got Yotsuha killed, he couldn't bring her happiness. For this, Fuu-chan resents him, they should not have been together.
(Assuming the year when Clover started grooming Uichi's sister was their arrival time and therefore what came even earlier is unchangeable, we can approximate to some degree what negative conditions both Yotsuha and Uichi may have gone through in this take of their lives)
In this timeline, Yotsuha had placed her four-leaf clover into some precious timepiece memento that's sturdier than the cheap watch she has now. Grasping this keepsake in her hands, Fuu-chan cries out to the heavens if she could go back in time and undo this mistake... and so she miraculously wound back to when she was in her 5th grade. (Think Ch 43)
(And so the Fuu-chan that Yotsuha knew has long been overwritten...)
Fuu-chan's mission is atonement for supporting the pair (as seen in Ch 53 and 61), revenge on the guilty Uichi and a prayer for Yotsuha to find happiness in some other way. How's that for a thematically opposite villain?
Furthermore, she needs Uichi to kill himself, so that he "willingly" doesn't want to be with Yotsuha and Yotsuha can let him go more "easily". Had Uichi been explicitly killed by someone else, then Yotsuha would still love him and spend her life mourning and hunting down his killer (zero time powers involved).
Would Yotsuha actually reach happiness without Uichi? Fuu-chan probably doesn't even know, though I won't be surprised if she'd have helped her find one after she succeeded in this "supposed" original timeline (ie what our Yotsuha believed to be the original), had Yotsuha not rewound time.
Limits
I'd like to think her "clock" being sturdier is what lets her travel so far back, in exchange for being one-time only. Making Clover a time traveller, but not a looper (or at least a voluntary one). She'd lack Yotsuha's ability to trial and error, but she compensates via the knowledge and skills she's acquired as an adult, like hacking, networking and psychology.
What I'm then curious is,
does Clover retain their memory every time Yotsuha turns back her watch? Is Clover an involuntary looper who piggybacks on Yotsuha's ride?
In any case, she assists Yotsuha with just enough hacking to stay in her good graces, and to learn what she knows.
Naming Schemes
As some of you are aware, all the named characters of this story are themed after numbers, with a few repeats. You've got cases like:
- Uichi/宇一, which means like "universe one"
- Satsuki/さつき (Hiragana-only), which homophones with 五月/"Fifth Moon/May"
- Rikka/六花, literally meaning "six flowers" (in reference to snowflakes)
- Nanami/七海, literally meaning "seven seas"
- Eito/瑛人, which rhymes with the English "eight"
Then there's of course Yotsuha/よつは (Hiragana-only) which homophones with 四葉/"four leaves" and Mitsuha/みつは (Hiragana-only) which homophones with 三葉/"three leaves". (On that subject, I bet their family has prior bad bloody history with time travel and Mitsuha will explain to her when finally forced to)
Notably however, we only know Fuu-chan/ふーちゃん by her nickname: her
real name hasn't been revealed to us at all! In Japanese mysteries you have to pay attention to literary tricks like these meant to obfuscate clues. Like, even for the longest while our suspicious Rei-kun/零くん we knew his real name contains a literal 零/"zero".
Going with the naming scheme of this story however, the most intuitive guess is "Futaba"/ふたば for 双葉/"twin leaves/sprout". Especially because her hoodie has a twin-leaved sprout on it, and her pigtail ribbons are tied like twin-leaved sprouts too.
And also, if my future Fuu-chan theory is correct, what happens when you overlay one Futaba/"twin leaves" with another Futaba/"twin leaves"? You literally have Yotsuba/"four leaves", also like a clover!
(As an aside, I noticed it's only in her teenage appearances that Fuu-chan sports her sprout-like ribbons. No ribbons shown in her elementary school appearances thus far, and no ribbons at Uichi's funeral)