Very late, but better than never.
Anyways, I've been working on this to showcase who scales to what multiplier-wise:
https://cdn.**********.com/attachments/814641582677426247/929057653487984720/KH_multiplier_final_hopefully.png
As I'm sure some placements may be out of place at first, I'll explain:
The ones at the very bottom don't really scale to any multiplier to begin with IIRC, that should be fine to keep as is.
Next up, Riku, Ansem and Roxas will clearly require three keys:
Riku: KHI-KHCoM | KHDays | KHII onwards
Roxas: KHDays | Post-Final Form Xion | KHII onwards
Ansem: KHI | KHCoM-KHDays | KH3D onwards
Ansem is a bit special, first, he scales to KHI Sora directly, then acts as the final boss of CoM to Riku, however, unlike Sora, Riku didn't have his powers reset, meaning that he kept becoming stronger constantly, and Riku sealed Ansem within himself, until he released his power to defeat post Final Form Xion Roxas. As Riku became stronger, so did his darkness, which is what allowed Ansem to become even stronger long term and upscale to KHII Sora by scaling to KH3D Riku.
Xion will require more keys, as she had an extra x1 multiplier per form, as discussed before, plus one for KHIII onwards as she can keep up with Roxas and Saix.
Xion: KHDays and First Form | Second Form | Third Form | Final Form | KHIII onwards
Then Saix scales to base late-game Roxas as he could keep a fight with him, then to KHII Sora as he becomes stronger to match him with the power of KH or so.
Saix: Base | Amped
Next up, the list is quite long, but fairly simpler:
Data-Sora: Comparable to KHI Sora, at least up to the point he got a real Keyblade, which is comparable to Mickey as together they defeated Sora's Heartless, and scales above Maleficent.
Sora: He's who the entire multiplier scaling comes from, so no need to reiterate the reasonings for them. In any case, KHI Sora is x1, and KHII onwards is x8.
Donald: Comparable to Sora constantly, so same as him.
Goofy: Same as Donald.
Axel: Can keep up with Sora in CoM and with (pre-forms) Xion in Days, his Absent Silhouette can fight Sora, Donald and Goofy in KHII, plus he did training in KH3D to keep up in KHIII.
Repliku: Scales to CoM Riku, which scales to KHI Sora, the second key comes from Xehanort's heart amping him in KHIII to keep up with others that already upscale to x8 KHI Sora (KHIII Sora, for example).
Marluxia, Larxene: Scales to KHCoM Sora, which is of the same level as KHI Sora, second key also comes from Xehanort's heart amping them to keep up with others.
Vexen, Zexion, Lexaeus: Scales to KHCoM Sora (Vexen) or Riku (Zexion, Lexaeus), which scales to KHI Sora, the second key comes from the Absent Silhouettes showcasing their capability to keep up with KHII Sora, Donald and Goofy, especially to remain consistent with Lexaeus being stated to be (unquantificably) above Xaldin.
Maleficent: Scales to KHI Sora, Donald and Goofy, second key comes from upscaling from Pete in KHII, who fought SDG, which fits considering she had some fighting between KHI and KHII in the events of Union X.
Aqua, Terra, Ventus: It may be weird for them to have this treatment, but I've noticed that it's more consistent this way, given that early on BbS they're on the level of (young) Lea, Hercules and Maleficent, and so they should be on the level of KHI Sora on the early-game of BbS, at around the part with Stitch (Which is before Neverland), they should be on x8 and all, which fits considering that the reason Unversed were around was to make Ventus stronger so he could keep up with Vanitas.
Xigbar: He should scale to x1 KHI Sora at around BbS to remain consistent with the scaling of Aqua, Terra and Ventus. Similarly, the second key goes late-game in the same manner by scaling to them.
Hercules, Hades: Scales to KHI Sora directly, then to KHII and so on, which fits considering we all know Herc keeps training to be stronger and so on.
Mulan, Mushu: Mushu scales to KHI Sora directly, alongside Mulan they scale to KHII Sora.
Simba: Scales to KHI Sora directly, then to KHII Sora while in his Lion form. KHIII is (unquantificably) above the KHII one.
Aladdin, Genie, Jafar: Scales directly to KHI Sora, then to KHII Sora. Jafar however only scales to KHI Sora, while Genie Jafar scales to KHII Sora.
Jack Skellington, Oogie Boogie: Scales directly to KHI Sora, then to KHII Sora.
Cloud, Leon, Yuffie: Scales directly to KHI Sora then KHII Sora because of the cups in Olympus Coliseum. If Aerith has a profile, it should be removed, from what I'm looking she's featless for our purposes and deserves an "Unknown" rating at most.
Beast: Scales to KHI Riku, which scales to KHI Sora. Beast can also keep a fight with Sora in KHII.
Pete: Fights Roxas and Xion in Days, then KHII Sora in KHII, also brings consistency to the post real Keybalde rating of Data-Sora, as he fights him too after that stuff in Coded.
Data-Sora: Pre-Real Keyblade | Post-Real Keyblade
Sora: KHI-KHCoM | KHII onwards
Donald, Goofy: KHBbS-KHCoM | KHII onwards
Axel: KHCoM-KHDays | KHII onwards
Repliku, Marluxia, Larxene: KHCoM | KHIII onwards
Vexen, Zexion, Lexaeus: KHCoM | KHII onwards
Maleficent: KHBbS-KHI | KHUX-KHCoded onwards
Aqua, Terra, Ventus: Early KHBbS | Late KHBbS onwards
Xigbar: Early KHBbS | Late KHBbS onwards
Hercules, Hades: KHBbS-KHI | KHII onwards
Mulan and Mushu: KHI (Mushu) | KHII onwards (Together)
Simba, Aladdin, Genie, Jack Skellington, Oogie Boogie, Cloud, Leon, Yuffie, Beast: KHI | KHII onwards
Jafar: Base | Genie
Pete: KHBbS - KHDays | KHII onwards
Now, for those that just scale to KHI Sora and nothing else:
Data-Riku: Scales to Data-Sora, who is comparable to KHI Sora.
Tarzan, Ariel, Ursula: Scales to KHI Sora.
Zack Fair: To remain consistent with the scaling for Aqua, Terra and Ventus, he should be on this level.
Prince Phillip: Scales to Maleficent.
(No need for keys here, lol)
Now, for those that just scale to KHII Sora and above:
Peter Pan, Captain Hook: Scales to end-game Terra, Ventus and KHII Sora, we already went over the KHI and CoM events being overall outlierish if anything.
Auron: Scales to KHII Hercules, which also scales to KHII Sora.
Master of Masters, Foretellers: Upscale from Player, who is by then at least on the level of KHIII characters as he inherits the powers of them with the medals.
Ephemer, Skuld, Brain: Scales to Player, who does the above.
Player: See above.
Odin, Hermod, Bragi, Urd, Vor, Baldr: Should at least scale to Maleficent given the latest trailer of Dark Road. That said, I'd rather hold on to the full release of it before doing profiles for them, as they can't scale to Young Xehanort either as that's after they've died or whatever.
Eraqus: Scales to Xehanort, and could put a fight to Terra.
Xehanort: Scales to Lingering Will, which upscales from KHII Sora.
Sephiroth: Scales above KHII Sora, although shouldn't be above Mickey per the statements.
Chirithy: Should be comparable to other Dream Eaters. Nightmare Chirithy could more than put a fight against Player right before the Keyblade War.
Meow Wow: Like other Dream Eaters, they come from Chirithies after they fused with the heart of their respective Keyblade wielder. Plus they scale to KH3D Sora and Riku, which are at least on the level of KHII Sora.
Neku Sakuraba: Scales to Dream Eaters.
Kairi: Scales to Lea, which scales to KHII Sora.
Yozora: Scales to KHIII Sora.
Mickey: Is stated to be the strongest character in KHI, and can keep up with Xehanort.
Stitch: Scales to KHII(I) Sora and upscales from Sparky, who could put a fight to late-BbS Terra.
Merlin: Is strong enought to be training Lea and Kairi, which scale to KHII Sora.
Yen Sid: Is regarded as the leader of the Guardians of Light, and trained Mickey. Could also notably help them in the KHIII Keyblade War.
Chicken Little: Scales to KHII Sora. Summons like him rely on a consistent power source from Sora, Drive Orbs, as it was discussed and accepted before.
Jack Sparrow: Scales to KHII(I) Sora.
Marshmallow: Scales to KHIII Sora.
Ralph: Scales to KHIII Sora and Player in Union X.
Xemnas: Scales to KHII Sora.
Young Xehanort: Scales to late-BbS Aqua.
Terra-Xehanort: Scales to Lingering Will, and is really just Xehanort in Terra's body.
Vanitas: Scales to late-BbS Ventus and Aqua.
Luxord, Xaldin, Demyx: Scales to KHII Sora.
Tifa: Can put a fight to KHII Sora in Olympus Coliseum.
Baymax: Scales directly to KHIII Sora.
(No need for new keys here, either)
Some characters may be missing, but this should help to have a consistent ground to develop on for other profiles. All Dream Eaters can scale to Meow Wow, Demon Tower scales to Mickey, etc.
I'm also aware that some of the above don't even have profiles yet, but futureproofing is wise to avoid issues in the future.
