My issue with Lucina is that her placement and relevance is riding almost entirely on Marth. She has done almost nothing to prove that she is a viable solo character and the only reason why people think shes good is because people know Marth is good
I mean, it's not like Lucina has all the exact same frame data as Marth or anything, or all of the tools that make his MU's good.
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It makes me laugh how people just assume Lucina is a straight up worse Marth/doesn't deserve to be classed as a good character/loses or does worse in MU's compared to Marth. The main excuses are because she lacks a tipper like Marth, or that she hasn't "proved" like he has done; this 2nd point is less of an issue, of which will become clear later.
When comparing Marth and Lucina, what we have to look at is how good they can both play against other characters; in effect, their MU spread. What's notable here is that Marth and Lucina's MU spread are effectively identical. One common misconception is that people rank Marth's MU's slightly better than Lucina's because he has the tipper and Lucina lacks it.
Marth doesn't win MU's because he has a tipper mechanic people.
It has never been this way in past games and is not the case in this game either.
Marth wins MU's because of two vital aspects about his character; range and frame data.
Example: Marth doesn't give Mario trouble in Melee and Brawl because he has a tipper mechanic. Marth does well in those game MU's because the combination of his quick frame data and his high range gives Mario trouble; getting in, off stage play, safely punishing etc - all of these are hard for Mario because of Marth's ability to threaten with a long ranged disjointed hitbox.
Marth doesn't beat Mario because he can get randomly tippered at 60% and die, for example; Marth beats Mario because he can succesfully limit what Mario can do thanks to his range and frame data. Tippers have minimal effect on how the main bulk of the MU plays out.
Let's put this into Smash 4 context: Marth still gives Mario trouble in this game; while his frame data has been made, shall we say, less dumb/oppressive, it's still fantastic (such as having one of the lowest overall landing lag/aerial start up totals in the game), and still does what it did in past games. Marth's range has been increased too, so the fact about range still stands true too. This is why Marth gives Mario trouble.
Now, let's look at Lucina. She's a clone of Marth. Lucina has identical frame data to Marth (before people say "but her range is shorter", it's a minuscule difference that has almost no effect on gameplay. Plus, you can still argue that her range is actually still higher than Melee and Brawl Marth too, so there's that). What does this tell us?
It shows us that as Lucina has all the things that allows Marth to win MU's, she can do just as well as he can.
The only difference between both the characters are the tipper mechanic Marth has and Lucina lacks. This simply means that Marth can potentially reap rewards earlier than Lucina can (and in the same vein, it also means he could "fail" [fail to KO] for longer than she can). However, no player is able to space tippers 100% of the time. As such, while you can argue Marth can hit a tipper Fair and KO at 90% near the ledge when Lucina cannot, you can also argue that Marth could miss-space that tipper off stage and not KO at 80%, when Lucina doing the same thing in the same situation would land a successful KO.
Essentially, the tipper mechanic rewards AND HINDERS players
in certain situations. Lucina lacking said mechanic does not make her an instantly inferior character that has no use in the metagame. The tipper mechanic
does arguably make Marth the better choice in the longrun (when you consider the important things such as KO potential), but it certainly
does not invalidate Lucina as a character, nor mean she has to prove as much due to how fundamentally similar she plays in MU to MU as Marth.
Tippers do not win MU's folks, which is why any MU Marth can deal with, Lucina can also deal with.
At this point, Marth should realistically be ranked higher than Lucina if only because his skill/reward ceiling is higher than hers. Everything else about the characters is fundamentally the same.
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Marth and Lucina discussion should not be avoided; there is far too much misconception about the pair that simply refusing to discuss them is backwards for Lucina's development. Discussion is fine as long as people are being civil, of which 90% of the time, it appears people are.