The main problem with Lucina is that, killing aside, she gets overall far less reward than Marth, in
many situations.
First and foremost, Lucina still needs to space her sword near the tip for her moves to be safe, like Marth. As such, Marth possessing the tipper mechanic makes this far more rewarding, because not only are his moves safer on shield because of this, he also gets more damage for actually landing these moves.
Going onto spacing, of their best spacing moves, being Dtilt, Ftilt, Fair and Nair, all of them (apart from Dtilt) KO at early/good percents when tipped for Marth. Lucina's versions of these moves, do not. This point is important, because, when at high percentages, it's no use having an Fsmash that can KO at around 80% like Lucina's, because it's just not a safe move. Sure, it KO's early, but unless you know it's going to hit, it's very unsafe to use (for both Marth and Lucina), due to it's endlag. As such, at the high percent ranges, Marth and Lucina should be playing a safety game, of safely tack on as much damage as possible before Uthrow starts to KO. Marth also has tipper Utilt which KO's at around 120%, and Lucina's doesn't KO until around Uthrow range.
That's all well and good, but, as I mentioned before, if Marth lands a tipper in these kinds of situations, he can KO much earlier than when Uthrow will start killing. This basically means Marth can KO off of spacing when Lucina cannot.
Now you might be wondering, "Well, Marth has a sourspot. Lucina doesn't have to worry about that when comboing, meaning she can deal more damage, right?". Well, in a way, that's right. However, what people like to gloss over is the fact that Marth can use his sourspots to combo into another move, when Lucina's blade (and of course, a Marth tipper) would launch too far for a follow up.
An example: at around 15%, sour Utilt chains into itself, then into an Uair, where Lucina can only hit 1 Utilt into Uair, as it launches too far for another Utilt to hit. Marth deals around 3% more in this situation, something that can be increased further if he tips his Uair.
Another: Marth uses Utilt twice. He can hit the opponent with sour Bair, and because of the low knockback, at certain percent ranges, finish with a Fair. Lucina, might be able to do 2 Utils if the percent is low enough, but her Bair launches opponents too far for a Fair follow up (and the minimally decreased reach of her moves has some small contribution into this). In this situation, Marth deals around 2% more, once again, which can be increased further if he tips the last move.
Another: This is simple; Sour Dtilt can confirm into Grab, Fair or Dancing Blade for a longer range than Lucina's.
In short, Marth can mix up his combos or options, based on whether he lands a sweetspot or sourspot, which is something Lucina for the most part cannot achieve.
Then of course, we have the facts, or proof. Marth has a bigger playerbase, and a much bigger results total. All this equates to Marth being a much more viable character than Lucina. That isn't to say Lucina is bad, so to speak, because, she does have a small few select matchups where she performs better than Marth, such as against
,
, and
(though from personal experience, I believe this to be questionable, but w/e, that's just my opinion). So theoretically, Marth mains, such as Mr. E and Pugwest should just be able to switch to Lucina to deal with these characters. However, the fact that these Marth mains
don't switch to Lucina for these matchups, and use Marth anyway makes me question how relevant this theory is. Perhaps Lucina can still more effectively fight them at close range, but perhaps the fact that Marth possesses the safer moves or more rewarding spacing game neutralises this theory? Whatever the case may be, Marth mains have shown us that they do not switch to Lucina, so I feel this point is becoming irrelevant. Pit/Dark Pit mains commonly switch between the two characters, so it makes sense for them to be grouped together. Marth and Lucina mains, do not, so why group them together?
Time for my biased opinion. I personally believe Marth
currently has potential to be somewhere within the 20th-25th range on the tier list. I think, that in time, it's possible he could rise ever so slightly above this, but he's not currently there yet. But, you see, I just don't see any justification for Lucina to be that high. I don't think she's by any means bad, but she has no real solid point to warrant using her over Marth. For that reason, I think she should be placed a little more than around 5 spaces away from Marth. I disagree with the idea of her being in bottom tier (for example, like ESAM's opinion). While this realistically makes sense when looking at usage and results, the fact that she is indeed somewhat similar to Marth holds a bit of weight. I personally can't accurately place Lucina, but if I think Marth is in the 20th-25th range, I'd at least say there are at least 10 more characters that deserve to be in-between them.