Here are some things that I noticed in your video against the Roy player.
Your movement needs work. It seems like you're not confident with it because I see you turtling a lot in your shield looking for shield grabs. A smart/fast player will easily be able to take advantage of shield campers. Work on your dash dancing and wavedashing for spacing (don't just do it for no reason), since that's one of Marth's biggest advantages on other characters.
Your approach is a little weird; well, when you're not just turtling. Multiple times I saw you start a combo or something with a dair, and that's definitely not a safe way to approach, considering the lag. You didn't seem to hit all of your l-cancels, but I guess that just takes practice. With that, you need to space your aerials or move out of the l-cancels so you don't get shield grabbed. Once you get better at this, you'll find that your opponents will have a much harder time getting grabs off.
This is something random, but you like to come down from above an opponent with bairs. I don't know how effective that is and I'm pretty sure it's safer to turn around with a fair or just land farther away.
Your edgeguarding needs work. As a Marth player, that's really important. Edgeguarding varies for different characters but for Roy, I feel like you should have been on the edge more. Roy likes to up-B from below so you can intercept that by dropping off the edge with an edgehop bair or dair. If they try to recover from above, waveland on the stage and do your fsmashes, fairs, etc. But the edge should be option number 1. Don't try to just randomly jump out and spike them, it doesn't work against most players and isn't really safe.
Hope I was able to help.
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