Hey, random user from the Little Mac forums with heavy experience for playing as Kirby, Marth, AND Lucina. If you're covering Marth, why not cover Lucina consider they are a bit different but not all that much?
I'll supply as much information for this as possible. This looks like fun~
Pros for 
Kirby has one of the best grab games out there due to his insane Forward Throw shenanigans, Back Throw being able to kill at very late percents and getting opponents offstage, Down Throw being used to launch opponents upwards once Forward Throw stops having links, and Up Throw having the capabilities to kill, especially early on stages such as Town and City as well as Battlefield and Dreamland 64, where there are higher up platforms that will make Kirby land closer to the top blastzone. Forward Throw is especially dangerous, with Forward Air being able to link after it and Up Air linking if they DI upwards, while there is no true follow-ups once past enough of a percent and them being able to airdodge, Kirby can just resume a combo by using Down Air. If they jump away, follow and Up Air.
Kirby can also Wall of Pain Marth and Lucina (Who from now on I shall name Marcina). Due to their falling speed being lower than a character such as Roy, who can DI down in order to escape or just DI away, Marcina have to eat up the damage or attempt to hit an
Marcina also have some slight problems killing outside of reads. Forward Smash works at the ledge if you, as a Kirby, aren't careful, Up Smash won't usually kill with Marth but with Lucina is more dangerous due to not needing a sweetspot, and Down Smash doesn't really do much on the front hit and only the back hit sweetspotted will kill oftentimes. Counter won't beat Stone, even if countering the falling Stone damage
Once Kirby copies Shield Breaker, the matchup becomes slightly worse for Marcina. Their Shield Breaker cannot hit you if you crouch, while Shield Breaker will hit them no matter what they do unless they spotdodge. However, due to the raw power that Shield Breaker carries in damaging shields most players will roll in or away, giving you the option to do a semi-Tech Chase where you force them into doing an action.
Cons for
If you get countered when using any Smash attack, Hammer Flip, or a copied fully charged Shield Breaker, you are dead. Period. Kirby's light weight doesn't help out much against characters with Counters, and the fact that Counter is yet another killing option means Kirby has to be wary of even trying to get a kill.
If you get thrown, most likely an Up Air will kill. Due to your low weight, this move gets added to the list of kill options and if they read an airdodge, kiss your stock goodbye from your count. Marth also gets Forward Tilt, Forward Air, and Up Tilt as killing options while Lucina gets a more reliable set of Smash attacks and Shield Breaker. Be wary of this.
You cannot approach just as your nature of a Kirby and being versus a disjointed character - Marcina in particular have a good time spacing you out and keeping you away. You must play patiently and wait for one simple mess-up, which most likely won't happen if your opponent is skilled enough.
Now for Marcina's - Kirby's Pros and Cons cover what they BOTH have advantages and disadvantages, but here is where they individually get their own sections. (Shorter due to me not wanting to work on stuff about Marcina anymore)
Pros for AND
Marth's Fully Charged Forward Smash tippered will ignore Stone's armor, no matter how hard you try, as it deals 25.2% and goes just barely over the threshold meaning it isn't safe to stay in Stone until he gets off the attack. Lucina does not, however.
Marth spaces better, which in turn keeps Kirby out a lot easier.
Lucina can deal with aggression MUCH easier and is better for having a get-the-hell-away-from-me resource, since her whole sword does good damage and not just the tipper. The Tipper does help, don't get me wrong, but the two of them
Lucina also doesn't rely on tippers for kills, meaning that her whole blade is used and used often for murder- I mean, destroying opposition. Forward Smash in particular is as good example of a move that is quite better due to an un-reliance on tippers.
Cons for AND
Marth in specific has some problems dealing with constant aggression, and while Kirby doesn't really have an approaching game he can come in close and begin stuffing out Marth and forcing him to get sourspots. His Sourspots won't kill Kirby until extreme percents, and when in close proximity players often make bad mistakes or just throw out attacks, meaning you can get off a grab against them.
Lucina is much more respectable in this regard, but she has even more problems killing than Marth. None of her attacks break Kirby's Stone, her Forward Smash is more often used as a get away tool and can become stale, her Up Smash has limited range, and all of her other offensive options barring a fully charged Shield Breaker or a Down Air Meteor won't kill until far past 150% without maximum rage.
Hope this helps.
