I got some time now:
Marth has a slight advantage against Ryu for several reasons. First, Marth's sword disjoint is good at keeping Ryu out when used appropriately. A simple weaving fair wall is problematic for Ryu to breach. Secondly, Marth has two excellent tools to intercept Ryu's Focus Attack/ FADC and landings with: jab and Dancing Blade. Both moves cut through FA like butter, and will put Ryu in disadvantageous situations (away/offstage with Dancing Blade & early % jab, and into potential KO scenarios with jab at higher %s). Thirdly, Marth has a great kit for dealing with Ryu's linear recovery trying to recover to the stage. Marth can occupy the ledge, drop off and bair wall Ryus trying to Tatsu back to stage. Counter can also do this as well as intercept Shoryuken. Lastly, Ryu struggles with characters who can end his stock early and bypass his good survivability to high %s(which averts him abusing his potent rage). Marth's tippers are excellent at accomplishing this, especially since jab sets up into, say, FSmash at reasonable %, resulting in dead Ryu much earlier than expected.
On the other hand, if Ryu gets in, he has a field day comboing Marth and his perfect comboing frame/weight, who doesn't have great "get off me" options at that point. Outside of jab, DB and grab, Marth is susceptible to FA (especially on predicted FSmash, USmash and Shield Breaker). Since Marth's aerials are almost all single hit, Ryu can escape juggle situations with FA/FADC where Marth tries to go after him in the air. If the Ryu player plays around jab opportunities/setups and Dancing Blade, and exploits Marth's poor up close range, it can go south for him very quickly.
For these reasons, Marth has the slight advantage, but it's not as bad for Ryu as you might think.
Will tackle Corrin (slight disadvantage for her) later.
Edit: Looks like RK's below advice got thrown out the window. You guys should seriously take a hint and drop it. Maybe FeelMeUp is right