Yep, Marth is kind of lame. He sucks in his game and his game is the worst of the franchise released in the west. He's only cool in Smash Bros.
I don't really see how Lucina is any less lame, if Marth's lame. Marth's bland. Lucina is bland. Marth has a cool design. Lucina makes that design female. They're both heroes coming from ruin. I really don't know what suggests her particularly that doesn't suggest Marth. It is not like the Awakening 'world of ruin' story and time travel is actually well done or she has a strong personality beyond dutiful, stoic hero. There is a bit of a wrinkle there; she is more reserved than a Marth or his clones, who're often more chatty.
I really think Lucina is sold to a) people who like reserved female, dutiful heroes and b) people who like the idea of a she-Marth/FE hero.
a) is often softcore sex selling it. b) doesn't alone speak volumes of her character.
I think there is a lot of a) in the fandom, and the more I see her handwaved as a good character, the more I think that's the case.
And I think that speaks to something FE has always Or at least long had. Softcore, inviting anime "sex." Awakening made that more accessible and attractive and player driven.
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As to Awakening, I imagine it sold in part as the anime/dating simulation itch for those who didn't want a more abrasively perverted product. My unit, the gameplay focus on pairing and children, the new art style, the complete abandonment of trying to tell a competent, if simple story, one-note characters whose tropes tended to be personality driven rather than narrative driven, etc. all sell this comfy sexy, but safe anime for the masses where you can turn your brain off.
Sure, casual mode helps a lot of people enjoy the game. And it helped Awakening sell. Doesn't mean the shift in tone, art, and narrative/character content didn't help.
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Edit: But on topic of the thread...I don't tend to get sold on sex alone. But...what I just wrote about Awakening comes partly from introspection (not that I like that game itself. I am fond of Lucina though) I do enjoy games that give me romantic feelings for characters I like for art, use, story, character reasons.
And then, it is taken with the whole package considered. Idk. I don't think it's clear cut.
Further edit:
This isn't really an argument. Fire Emblem is at forefront of the Japanese SRPG currently because Int Sys approached the game to appeal to newcomer explicitly after the poor production values of New Mystery, Shadow Dragon and Radiant Dawn.
While Radiant Dawn was a pretty mediocre looking Wii Game with some good looking CG. Awakening was one of the 3DS games with some of best production values at the time and made an incredible case for the idea of having an anime entirely in 3D CG.
Compare the marketing for Radiant Dawn and Awakening and it's night and day.
The argument of Waifus is nebulous at best. It was production values and some brilliant marketing that sold FE and made it into the face of the genre currently. Then the relationship systems were spread via word of mouth. And both Men and Women were appealed to. FE has a substantial Female demographic as well as male demographic which has not changed even today.
Of course it is nebulous, but about the bolded, sex sells to women too. Just look at female gendered entertainment. A lot of it is about hot, dream guys at some level, and often the chief level. Look at shojo. Look at "trendy" KDrama. Look at boy bands. Look at a lot of young adult fiction.
A lot of this stuff is fairly softcore, just as it is in FE.
Or is softcore sex helping sell something not sex selling something?
I think when you've got a game that makes itself more sexually accessible, interactive, and presentable, and when this is an oft toted feature of the game among its fans, it probably helped sell that game.