Kafei? That would be very interesting, actually. Didn't people speculate that he's actually Termina's Link, since he's one of the few people who didn't have an equivalent in Hyrule? And he shared Link's bones and animations, as well, I think. He'd be a good alternative to Skull Kid for representing Majora's Mask, and would be easier to clone since he would essentially be Young Link. That's also the biggest problem, though. He would be just like a third Link, unless there was some way to distinguish him from the rest. What items are in Majora's Mask, other than the masks, that aren't used by Link or Toon Link?
Kafei is known to always be wearing a mask and was even temporarily playable during a sidequest. He uses hand to and combat translated varients of Link's moves in MM. Using the non-transformable masks can provide much more variation than you would think. Even then, you can also implement the Great Fairy Sword into his moveset as well. It was mentioned in the story that Kafei did go to visit the Great Fairy to find she had been separated into stray fairies. It can be very well implied that Kafei would have attempted to stop Skull Kid himself. Even if it would have ended in failure. In short, he lacked the tools Link had at his disposal to do the job himself.
To the point, items like the Blast Mask, Mask of Truth, Bottle, Magic Bean, Sling Shot, Deku Nut, Bomb-Chu, and Great Fairy Sword are all items that would be feasible for Kafei to use.
The Blast Mask would be a risk and reward move that could also benifit as a recovery if used properly.
The Mask of Truth, if possible, can be used for a counter.
Bottle could be used for many things. Reflecting a projectile as a nod to the ongoing gag in the Zelda series, perhaps a potion or
Healing based item to accommodate for the blast mask damage. Still risk in useage and heal factor would be balanced accordingly.
Magic bean would be an idea for an Up-B. Just a suggestion, but one I personally don't prefer.
Sling Shot can be similar to the discarded VBrawl Toon Link Arrow, or maybe a rapid fire move that doesn't flinch the target like Fox's laser shots.
Deku Nut is pretty self explanatory. Would only stun grounded enemies. KB would be weaker Tinks bombs.
Bomb-Chu would be an interesting combo tool and very good for pressure/set-ups.
And Great Fairy Sword could be used for his smash attacks or another option for his up-b. Maybe both.
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So feedback on which of these moves would be preferred or feasible would be nice. Suggestions would also be appreciated. I personally feel bombchu or blast mask, great fairy sword, deku nut and sling shot are the best options here.