Updated: Oct 18, 2021
I chose those words carefully last time because this set is more than just a "pairing":
WII FIT TRAINER
--Unlocks: #9 in Vs, #2 in Samus's Classic line (after Inkling), or in Upper Consoleville in the Light Realm of WoL.
--Unlocks: #6 in Vs, #1 in Pikachu's Classic line, or at the crossroads in the Light Realm of WoL.
--Faster & lighter.
--Pocket is now indefinite & shows up on the HUD.
--Unlocks: #20 in Vs, #3 in Mario's Classic line (after Sonic & Bayo), or at the Gym in the Light Realm of WoL.
--Faster & heavier.
--Jolt Haymaker no longer triggers helpless fall.
--Rising Uppercut goes higher.
--Giga Mac is now Giga Mac Rush.
--Unlocks: #22 in Vs, #2 in Pikachu's Classic line (after Villager), or in atop the waterfall in the Light Realm of WoL.
--Monado Arts has a faster interface & the timer shows up on the HUD.
--Mecha-Fiora is part of Chain Attack now.
--Unlocks: #48 in Vs, #8 in Mario's Classic line (after Sonic, Bayo, Mac, Ike, Luigi, Roy, & Doc), or in the mushrooms in the Light Realm of WoL.
--Pikmin are more vulnerable.
--Grabs are stronger with blue/more Pikmin.
MR. GAME & WATCH
--Unlocks: #52 in Vs, #6 in Pikachu's Classic line (after Villager, Shulk, ROB, Mega Man, & Isabelle), or in the maze in the Light Realm of WoL.
--Fair is now a bomb drop.
--Chef can now be aimed.
--Judge cannot repeat numbers consecutively.
--Oil Panic reflects non-absorbable projectiles.
--Octopus is no longer controllable.
--More historically-accurate animations.
--Turns characters into their stock icons when juggling.
--Unlocks: #11 in Vs, #1 in Fox's Classic line, or at the racetrack in the Light Realm of WoL.
--Altered first jab & f-smash.
--Knee of Justice has a bigger hitbox.
--No longer stabs with battering item f-smash.
--Raptor Boost doesn't trigger helpless fall if it connects.
--Unlocks: #31 in Vs, #3 in Pikachu's Classic line (after Villager & Shulk), or in the Mysterious Dimension in the Dark Realm of WoL.
--Faster & smaller.
--Down throw buries.
--Robo Burner has more fuel & shows a fuel gauge.
--Dropped Gyros can be re-charged.
--New FS: Guided Robo Beam (source: Ultimate)
--Unlocks: #56 in Vs, #8 in DK's Classic line (after Bowser, Red, Rosalina, Dedede, Sheik, Greninja, & Diddy), or in the poison woods in the Light Realm of WoL.
--Faster & lighter.
--Trick Shot can now be shot in the air.
--Clay Shoot now explodes into shrapnel.
--Duck Jump can cancel into an attack or dodge before helpless fall, & it can be reused after a windbox.
Guided Robo Beam is definitely a step up over Super Diffusion Beam, but I still prefer the original Diffusion Beam.
And remember, "I'm-a Luigi, number one!"