This theory right here is probably why I cannot get the whole Corrin is OBVIOUSLY 3rd best FE, and has "poor theory" trend that was all over here. Frame data, combos, mobility, and safety are key to a good character. I whole heartily agree with this. Maybe this belief stems from my previous experience as a street fighter player (honestly somewhat true for all fighters, but especially in SF). Honestly though let's look at each FE characters individual stats in all the previous categories objectively ( Mobility, dmg racking, kill power, start up, and shield safety):
Frame data:
Marth:
https://docs.google.com/spreadsheet...WFwy_dg04Rrq4a-YF9-VDOhZI/edit#gid=1636352940
Ike:
https://docs.google.com/spreadsheet...WFwy_dg04Rrq4a-YF9-VDOhZI/edit#gid=1747190514
Corrin :
https://docs.google.com/spreadsheet...OWFwy_dg04Rrq4a-YF9-VDOhZI/edit#gid=943524191
Based on data lets go through each category of: mobility, dmg racking / combos, kill power, Move start up, and lastly Shield Safety. If you disagree feel free to state something I may have missed, or your interpretation of the data. I will try to be as objective as possible (though me being a Corrin main you can view this with slight bias if you like). Also Combo / kill categories were hard to do on data alone ( did based partially off matches seen).
Marth V
Corrin:
Mobility:
This easily goes to Marth. Corrin only has her initial dash, better b reverse, and dash to shield on Marth, the rest is a resounding Marth.
Dmg racking and combos:
Corrin really shines in this area among the swordies. Her combo game can convert from the vast majority of her moves. This is even even at their safe ranges. They are also consistent in dealing large chunks of dmg. Marth on the other hand has combos, but not near the lvl of consistency or dmg of Corrin's. The Marth tipper's do nice dmg, but have large knock-back and are difficult to get follow ups on. This is very much In Corrin's favor.
Kill power:
I personally am very hesitant to put a verdict here. I'm inclined to put Corrin due to her consistent ability to kill multiple ways and at multiple times, but I also cannot deny Marth's ability to get early kills off his amazing edge guards, and his tipper smashes (both of which are just above Corrin in their potency). I'll honestly call it decently even with an advantage to Corrin for her consistency in killing when compared to Marth (While also having early kill power). Corrin Having a better kill throw, more kill moves, and not needing to land tippers every time is why I say this.
Start Up:
Start up on their moves are pretty much the same. Honestly they almost copied the frame data over from Marrth for Corrin. Yet, there are some advantages in start up which go to Marth. His smash attacks, and U-air are slightly faster than Corrin''s. Also his F-air is 1 frame faster than Corrin f-air, but not her N-air, which Corrin uses more often. (honestly both their frame data is amazing for Swordies). Corrin's Tilts come out faster but only by a frame or 2. Honestly pretty even.
Shield Safety:
For safety of aerials in general Corrin has Marth beat. For Marth to be comparable or slightly safer he needs to tipper, even then some of the shields advantages are in Corrin's favor or even. The Shield advantages that are better for Marth are 2nd hit tipper n-air, and tipper U-air, the rest is Even or better for Corrin without need for tipper (for example tipper Marth F-air is -8 tipper ed, Corrin N-air is -7). The ground though is in Marth's advantage. With Marth down tilt being a stand out -10 to -9 tipper ed, his other tilts though are near double that. Corrin's around double marth's down tilt as well (Corrin -19 d-tilt). Marth also has slightly better ranges on his ground moves. Gonna give this being close, but to Marth.
From a base look its pretty even in most categories, and the only true big gaps come in combos, and mobility. Honestly It may come up to personal preference in the future between choosing these two. Though my own Bias opinion tells me Corrin's better kill and dmg ability, while also being almost as safe would win out.
Ike V Marth (Not as familiar with either compared to Corrin, will use mostly data, and will be shorter)
Mobility: Ike is beaten by Marth in probably every category here. Not much to say
Dmg racking / Combos:
Ike has a good dmg raking game when compared to Marth. His angles combo better, he has a combo throw, and he hits like a truck. Marth has his aforementioned tippers, and some combos, but they still can't make up for the raw dmg and combos Ike has.
Kill power:
Both Ike and Marth can kill very early. Ike has a large amount of kill moves without the need for tipper. Marth on the other hand has a ok kill throw, and a better edge guard game. Gonna call it even.
Start up:
Marth, really just Marth. Only Ike's jab is faster by one frame. The rest is either even or largely in Marth's Favor.
Shield Safety:
This is about even. In the air Ike's aerials generally a little safer than most of Marth's aerials with -7 on shield. This is followed by Ike's U-air which is also -7 on shield (but less range). Marth to be comparable needs tippers in the air. Yet, there is still u-air tipper and 2nd hit n-air tipper (but these are rarely used on sheild compared to say tipper f-air). On the ground Marth is generally a little safer with his tilts. D tilt on Ike is -15, while Marth is -10 / -9 (tipper). Ftilts are -19 Ike, and -18/-17 Marth. Its really Even here.
This seems somewhat in Marth Favor, as he wins in mobility, and start ups. Yet, Ike like Corrin beats Marth out In combos and dmg racking. In the other Categories they seemed pretty even.
Corrin V Ike (less data in text because I've already gone over data for each in previous ones).
Mobility :
Sate wise it was literally even pre 1.15. Then Corrin had a 3% decrease on base mobility stats vs Ike. Yet, Corrin has some things going for her. One a good B reverse / wave bounce. Two better initial dash, and dash to shield. Three, side B off platforms (which auto cancels about 2/3 the way through... you can b reverse, n-air, or jump before landing, landing with nothing is a 1 frame soft landing). Honestly Corrin just has a couple mobility tricks over Ike and its near even.
Dmg Racking / Combos:
Both have great combos, but Corrin follows up into juggles better, and does not hit the target to far away. She also true Combo's to higher percents. Vice versa Ike has a combo throw, and hits more like a truck ( It was too big to be called a sword... more like a big chunk of Iron). Pretty even.
Kill power:
Ike hits like a truck, and kills like truck. As far as power is concerned if Ike is a semi, then Corrin is but a pick up truck. The thing though is that Corrin has a good kill throw, and sets up into her kills better. Its probably in Corrin's favor with her kill throw, and kill set ups. She does not to read as hard as Ike to kill.
Start Up:
Corrin is comparable to Marth here, so like him she trounces Ike in start up if it's not b-air or jab.
Shield Safety:
Ike like Marth is safer on Shield. Similar aerial safety, and has slightly better ground safety whe compared to Corrin. Ike is just a little bit safer than Corrin.
Honestly Ike has three things in these categories better than Corrin. Shield safety (slightly but there), hit for hit power, and a combo throw. Corrin on the other hand has much better frame data, true combos / juggles, and has a kill throw.
Honestly this is how I see the data. Feel free to posts any constructive criticism, or praise :3 (damn that took a while).