Kayako is not quite as strong as the Leafe Knights though is able to shake an apartment putting her at least somewhat relative. Speedwise she can move faster than the eye can track putting her at similar speeds.
Kayako can move around invisibility and inaudibly, meaning even Sasame and Kei wouldn't be able to detect her if she didn't want. Kayako has absorbed the souls of everyone she's ever killed, hundreds of people, and can summon them as zombies, duplicate herself for each one, or create illusions. This would be absolutely cruel to use against the tier as they would be fighting a seemingly never-ending wave of zombie/ghosts some of which they can't tell where they are and which can one-shot them by absorbing their souls. Kayako has teleportation across dimensions, dramatically better than the tiers meaning if they tried to BFR her, she could simply BFR them in return.
If she wanted to she could even possess them or turn them murderous to have them fight each other, not that she particularly needs too. And obviously the Saihi Seeds wouldn't be able to do much against her since she is a dead Onryo. Hypothetically their leafe drain would eventually kill her, but that's way too slow given how quickly she can destroy things like living evil trees.
The only weakness is if she overconfidently attacked them head-on right at the start the 7 could seal her, and if they did she might not be able to escape since she is dependent on the Demonic Abyss, and wouldn't be able to survive 16 years without it. They're a lot more skilled fighters. Sasame and Kei might be able to sense her as well depending on how much she focuses on being stealthy. If she's flying Hayate might also be able to sense the change in air currents or Mannen might be able to sense her cold presence.
Kai Parker from the Vampire Diaries
Kai Parker as a Witch/Vampire hybrid and one of the strongest vampires in the world, scales above the vampire Marcel who withstood an explosion comparable in size to a small building. Vampires are also fast enough to simply appear next to people as though they had teleported. Given that normal vampires are similar in stats, if not faster, than the Leafe Knights, Kai should have a signifigant stat advantage over the tier.
Kai was orginally a Siphoner, someone who can sense and absorb the magic of others which could mess with the Leafe Knights dramatically be weakening their abilities. He can create a vampire army by infecting people and has the stealth+intelligence to do so. He has invisibility-based magic and with simpel hand gestures can create loud disorienting sounds that mess with people that sense by sound like Sasame. He is a massively skilled manipulator sociopath that could blend into societ regardless while the Leafe wouldn't know where the threat is coming from.
He also has his own elemental manipulation to get around the Leafe's blasts and barriers, can use sleep manipulation which is pretty dangerous considering how few fighters the verse has in general, and has illusions that can help wear down the Knights' leafe reserves. Beyond that he dimensional travel so strong as being able to bring him and others outside and inside a prison dimension suggesitng he could get around the Leafe' sealing ability. He also many abilities to aggravate further pain and exertion such as telekinetically attacking peoples organs or inducing excrutiating pain making it even harder for them to use their magic. Overall Kai would just be an absolute nightmare for the verse. Even if they found out his identity, he has memory erasure to resume his secret identity.
Planetary Scale Threat:
A girl of legend is born, the Pretear, one who can create new leafe, who can imbue the world with life and spirit. However the previous Pretear fell to darkness and hatred, becoming the princess of destruction Saihi Fenrir. This tier contains the current Pretear Himeno Awayuki and Saihi Fenrir herself.
The two in terms of raw power scale to multi-city block to town level scaling from Saihi granting one of her seeds power to cause an earthquake, and causing unnatural red snowfall with her actions. In terms of speed, Saihi Fenrir is made of darkness. As darkness is the absense of light, this should scale the to lightspeed exactly along with arguable scaling to Kei's light.
Saihi Fenrir's physical body is still sealed away. However after 16 years she was able to spiritually project, and its her spiritual projection that is really being fought during the series. As such it has spiritual existence. Her most distinctive force gained when her heart fell to darkness. Instead of emitting energy, Saihi Fenrir absorbs leafe within her, draining the world of its energy.
This is by far her most dangerous power as its strong enough that it was stated she was going to bring about the end of the world all things, with leafe forming the foundation for matter itself. It is stated that her power would leave the world an empty silent void without temperature. It's strongly implied this would not just be planetary but universal as not even temperature or light would continue to exist, and contextually its clear that the Leafe Knights couldn't avoid this by going to Leafenia. As things derived of all leafe disappear no longer exist as it forms the foundation for all things this is essentially universal scale energy+life draining and existence erasure.
After her possessing a civilian to get a body, she caused an unnatural red snow with her powers said to bring calamity, though what that means is unclear. It began draining the leafe from things making it hard to see, to hear, to feel things derived from light, sound, and temperatures. She also transmuted the trees of the city into her Saihi Seeds of the prior tier. These trees can do things like "eat" all the sound leafe in an area so people didn't know Himeno was in trouble.
However she was stopped by the pretear of the current generation, Himeno Awayuki. As a Pretear, she has the power to create new leafe, new energy. After awakening to her power this gives her a functionally infinite stamina as she can create new energy for herself. Initially she had no idea how to use her power and had to fuse with the Leafe Knights so she could create leafe to power their elemental magic. However she eventually awakens to her form as White Pretear, giving her the powers of all the Leafe Knights, as well as access to the fullness of her leafe energy. During her battle with Saihi Fenrir, as Fenrir tried to absorb all leafe, Himeno embraced her clashing her energy creation ability with Fenrir's energy absorption, the two actually perfectly equaling out suggesting Pretear's spiritual energy creation reached similar universal levels before Pretear purified her.
Both of them also have similar power, flight, and Himeno was able to tell someone was watching her showing minor enhanced awareness. Himeno has purification that was able to reach Saihi Fenrir's buried love and defeated her. Himeno is also a martial arts master physically relative to the Knights before gaining her magic.
So how to fight this tier? It's definitely intimidating to fight someone at town level whose a spiritual entity with universal erasure + energy drain. Well her powers definitely seem to work on loving things faster, as it is specifically draining the life from things, as such a non-physical or non-living counter would give at least some resistance. Beyond that purification was what worked on her in canon so a counter with purification should work here as well.
What also is notable is that there are only 2 fighters on the tier, and both are of the emotionally unbalanced sort, Saihi Fenrir being a love-mad vengeful spirits and Himeno being the impulsive sort prone to not thinking things clearly. What that means is manipulation, either brainwashing or even just general manipulation would likely be much stronger than normal as it only has to work on 1 person to control half the tier, and on both it would just outright solo. Himeno especially wouldn't want to fight something cute. Which brings me to my first counter
Fluttershy from My Little Pony: Friendship is Magic
Fluttershy is arguably the second fastest of the Mane 6 able to outpace when heavily determined a slightly weighed down Rainbow Dash who is much faster than Twilight Sparkle. Twilight Sparkle for comparison was able to light-time, putting Fluttershy in the speed range for the tier. Fluttershy is also stronger than Spike who dispersed a large ice cloud with his fire breath putting her in the general range for the tier.
Fluttershy usually avoids direct fighting preferring to befriend enemies or to stop them with her technique "The Stare", an intimidating stare that compels people to stop fighing. This would very likely allow Fluttershy to stop Saihi Fenrir given her psychologically unstable nature. From there it would be pretty easy for Himeno to beat her assuming Himeno prefers to target Saihi Fenrir over Fluttershy which...is pretty hard to argue. Not only would the Stare probably work on Himeno as well, Fluttershy just in general has an alright chance against Himeno in a fight even if she's not as versatile or skilled.
However given that human world Rainbow Dash was able to briefly tap into her was able to tap into one of her elements it's likely Fluttershy could tap into her own element, an element strong enough to seal the spirit of Chaos Discord and therefore should likely be able to seal Fenrir as the Leafe Knights did, as well as undue effects targeting Fluttershy. Like the Elements of Harmony on the whole are stronger than even Himeno's Leafe Creation suggesting if she could use the Element of Kindness Fluttershy could replicate Himeno's exact strategy against Saihi Fenrir, if not better and just overpower Himeno.
That said without the Element of Kindness if you don't think she can use it Himeno probably beats Fluttershy due to superior versatility and skill, and Fluttershy would be endangered by Fenrir's abilities if she gets them off. For more of a warrior counter you could use
Limone from Wedding Peach
Limone is a warrior angel and should at least be comparable to the illusionist devil Arushion who created a false dimension which stretched to the surroundings and Momoko's henshin into Wedding Peach which destroyed the aforementioned devil. The battle between Angels and Devils in Wedding Peach regularly uses light and darkness attacks against each other, scaling them to the same speed as the tier, if not much higher as more powerful angelic beings have feats of traveling and projecting light between worlds quickly.
Limone uses holy spiritual power, a purifiying power that would absolutely work on Saihi Fenrir considering WP-verse has way better purification feats than Pretear. Saihi Fenrir is ultimately a misguided human while the light of the WP-verse angels can work on devils, literal spiritrs of hatred. Momoko even in her civilian form was able to purify a lesser devil with her sheer presence suggesting Limone might purify Saihi Fenrir with her presence. Likewise her even trying to fight him may not work as angels can purify the energies of malice and hatred. Plus he can travel and bfr through dimensions traveling often from Angel World to the Human World and even the Devil World, which is something that against the Leafe Knights Saihi Fenrir actually somewhat struggled with.
Against Himeno Limone is a far more composed warrior being characterically calm and devoted as opposed to tempermental and his ability to combat specifically malices may be able to keep her from attacking him just in general. Beyond that while Himeno is a master fighter Limone is a warrior angel who has fought devils for over a century, and can create angelic barriers strong enough to protect against some of the strongest devils in the series who are in the planetary range, meaning it can easily block all of Himeno's attacks.
The only threat for Limone is that he is pure spirit or in Pretear terms pure leafe. It's unclear whose passive presence would work first; Saihi and forces leafe drain or Limone's purification, though I would personally bet on Limone and if they start at far enough range and Limone doesn't avoid through other dimensions Saihi could absorb the leafe of the universe which would work on Limone. For the ultimate counter you could use
A Fuzzy from BlazBlue.
Fuzzies are a series of units made from the remains of the Black Beast. While Unit 16 was able to fight relatively early Ragna fairly evenly, the stats of early Fuzzies are unknown and could be interpreted dramatically lower, dramatically higher or, as I'm using here, actually even scaling off an early attack from Tager calced at 900 kilotons. Given Fuzzies were able to fight this Tager even when highly weakned you could scale this hypothetically to early Fuzzy Units. Likewise light-timing also exists for even low tiers in the verse, suggesting a Fuzzy would have a notable stat advantage but be fair to use.
Fuzzies are artifical half-vampire units meaning they would be completely immune to all leafe based attacks like Saihi's leafe drain. This is troublesome as Fuzzies have regen that allow them to supposedly regenerate from any wound, essentially requiring a hax to work on them something the verse would be extremely hard-pressed to find. Likewise in combat they summon armies of the undead, similarly immune to leafe-based attacks.
But couldn't Pretear purify them? ... Highly unlikely. Fuzzies are remanants of the Black Beasts and are massive sadists. They're clearly way more corrupt than Saihi Fenrir which took all of Pretear's power to purify.
However that doens't bring up the real threat that the Fuzzies possess. Fuzzies are stated to be able to control not just skeletons and ghouls as seen but also spirits. Even assuming this doesn't work on Saihi Fenrir since she is still technically alive, it's implied the Pretear line goes back a long time, and it's possible the Fuzzy could raise an army of undead Pretear...given their hearts would be turned to hatred this would essentially be like summoning an army of Saihi Fenrir.
This is not even adding in the possibility they may scale to any of the more insane broken hax in the verse that they might being literally pieces of the Black Beast. Even without that though, they are immune to almost every ability in the verse, would be extremely difficult to defeat conventionally, can summon an invulnerable army of characters each identical to the villain which is one of the two tops, all while flying around and one-shotting basically anyone.
And that's how to be OP in Pretear.