Okay, just a few things I noticed.
The Marth feels awkward. It looks like he's trying to control the match and space the Fox out, but he's walking in too close. It also seems like he's not confident in his own timing - there's a lot of weird pauses before he does something, leading to his attacks failing. Technically speaking though, I think he's doing well, although he's rolling sometimes when he should be wavedashing and his DI could use some work - he's just going straight up from uthrow uairs and just in general it doesn't look like he's DI'ing too well.
It doesn't seem like he's following up on any of his hits either. The chaingrabs were goodish, but he didn't really have any combos. I think it's because of his move choice - if he replaces some of his nairs and uairs with fairs, he'll have more options to combo with. The dtilts also don't seem to be doing much, nor are they connecting, so he should try another approach option.
Speaking of approaches, it doesn't look like he's JC'ing his grabs.
Oh and two other things he should work on are the recovery and the edgeguard. I'm guessing he's a little nervous because of the shinespike, but he's recovering way too high and using his double jump way too soon. He panicks a lot and messes up the sweetspot - there was also one time when he didn't even try to recover from a high shinespike even though he still had his double jump. If he covers his recovery with fairs, he'll get shinespiked less and survive more.
The Shield Breaker edgeguard isn't working out for him at all. Charging a Shield Breaker severely limits Marth's options and most of the time it won't get stronger than an fsmash. He should try chasing Fox out with fairs or fsmash/dtilt. Maybe go for the dair spike every once in a while.
Yeah, that's about all I have for now.
