I suppose this is a bit late, but I'll post this for whoever else is reading.
TL;DR: Three good characters to choose from when trying to "go easy" on newer players are
![Bowser :bowser2: :bowser2:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
,
![Jigglypuff :jigglypuff: :jigglypuff:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
, and
![Fox :fox: :fox:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
/
![Falco :falco: :falco:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
. (details on how to exploit their traits for the sake of teaching new players fundamentals are below)
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For those of us who find it hard to lower their gameplay level and "go easy" on newer friends, Bowser can be a good character to play as. Bowser is slow, has bad vertical recover, has short to average range, no projectiles (excluding the
slow fire breath), and has a massive hitbox. Because of these traits, the timing required to land hits on Bowser is much more lenient than with the rest of the cast. This will help to develop their understanding of basic combo and kill setups before they generally would when facing other characters. At the same time, you should use Bowser's good aerial options and brutal one-move punishes
(on a basic level...e.g., a dash attack or side-smash could tempt them to try interrupt it since it seems so slow, but the deceptively long-ish range and super armor punish thoughtless approaches) to give them a sense of slower and extremely apparent reads. You could tipper F-smash them with Marth, but because it may seem so quick to them, they might not even take notice to how it was used as a punish; brand new players often just go into auto-pilot 90% of the time. Bowser's F-smash, on the other hand, has a super slow and distinct startup movement, and dying early from it will teach them to be more wary and look out for it. Don't use it as much immediately, but once they have a basic comprehension of how to properly move/attack with the character in the way they want, use it a few times per match. If they don't seem to be trying to adapt to it, use it more often and punish them hard with it. They'll start to more consciously think about baits/punishes/reads in play.
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If you feel that you already have too much experience with Bowser, or you're generally accustomed to playing slower characters, try Jigglypuff...but take care to
not go for B-air spam and her other crazy gimmicks. Instead, play her like you would with a defensive Marth; try to catch them on the tip of each of Jigg's attacks, maximizing the spacing between you and your opponent so they can't hit you. Of course, that's probably the stupidest way you could go about playing a character that depends on you spamming her moves with superior priority and startup time, but playing anything resembling the way you 'should' play Jigglypuff would be sure to frustrate them, and deter them from the game. That's not what you want. On that note, try to avoid using aerial attacks completely; just stay grounded. Jiggly's horizontal air movement god-tier on nubs, as is Rest (so avoid using Rests unless they find it funny).
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If you're good with both of these characters, but aren't as comfortable with faster characters / aren't very tech-heavy, go for Fox/Falco. Both have easy-to-learn kill moves as basic but obvious punishes (Fox's U-smashes/U-airs, Falco's spike). Their odd low-level playstyle will likely hinder your play if you're not used to playing similar characters, allowing them to take advantage of your awkward movement and short attack range (minus lasers). Projectile spam can also force them to practice approaching if they begin to play too defensively for their own good. Again, it's important to play fox/falco how they shouldn't be played (concerning mid-high level play; low level play, like F-smash / Dash Attack spam, will encourage spacing awareness and their reads/punishes), just like with Jiggs. Both have a lot of neat stuff to learn about, but that's not what you're trying to accomplish here; make a conscious effort to play the 'bad' way.