This is only from watching the first video. (I'm on dial up right now, so it's difficult to watch vids)
You need to work on comboing (starting/maintaining/finishing) properly. He is vulnerable coming down from the air several times while still at low/mid percent and your reaction was to get in a fsmash rather than start a combo which you could end with a fsmash or better while racking his percent up first. Also, shieldbreaker vs a 0% opponent isn't a great choice just because its difficult to combo out of and they don't have much hit lag. Basically it's risky and will likely get you punished regardless if you hit them with it or not.
Your DI needs work. The ken combo on your second stock could have been stopped with gradual consistent DI or at specific points as well, such as when he utilted you.
Your overall reaction speed seems kind of slow. One big thing you can do is to completely shift your point of focus to watching the opponent. You need to be comfortable with the character and with everything you can possibly do while maintaining a good sense of how you are moving, but if you are constantly watching your opponent, it becomes much easier to react to what they do. It seems like you are missing a lot of openings because you just see it that half second too late.
You both go for dolphin slashes at early percents. Knowledge of what your characters moves can do and when you should use them is pretty important. You were at 0% and he was at 50+%. You don't want him to be far away from you because you (should) have the advantage in offense at this point. Any move at this point can combo pretty decently, plus you were facing the middle of the stage far from an edge. All this is going to do is give him time to think, not a lot of %, and not even knock him to the edge of the stage.
You use that airdodge to the edge a lot and he killed you for it eventually. It's ok to train the opponent by using the same recovery method a few times, but that specific one is much easier to counter, especially at the height you are using it at.
You are retreating from edgeguarding more than you should be, especially in a Marth ditto. Marth is one of the best edgeguarding characters, and you have no reason to back away from another Marth being on the edge. If you stand about the distance away where you can't quite tip the edge if you were to forward smash, you can almost perfectly react to any choice in getting up by dtilting/fsmashing/grabbing/fair/whatever. Just be patient and wait for them to come up, don't try to force them to. If it's really a problem, just walk slowly towards the edge and dtilt the edge. They will do something, and dtilt has minimal lag, letting you punish them for whatever they do, or back up again if they drop down and regrab the edge
Forward b combo in Marth ditto isn't great either. If anything, just use the first fb and depending on their DI/% utilt or grab. Or if you are just trying to be a ****, keep hitting them over and over with the first fb until they DI out of it. (This has been dubbed the "Cactuar" Combo (aka "1,000 Needles" if it's 3+ forward b's or "10,000 Needles" if you end the 3+ fbs with an utilt kill) by Mew2King. It's a joke combo, only to be done when you are at a large advantage over your opponent. It is easily DI'd out of, works around 110%+ and can be ended easily at 140% with an utilt kill if they really want to die.)
Also, continuing with my "keep your eyes on your opponent" thing, you could have avoided losing that last stock so easily just by watching what he was doing at the edge.
Anywho, this is a pretty nice improvement from the last vid I watched of yours. I'll try and add more after watching the second and third vids. Keep it up
