@fitizen
You're a squalid manchild with no taste and you're using racially derogatory shorthand when you're completely capable of spelling 'Japanese' appropriately and not even taking more than half a second to do so, no doubt. But, your insult and my response to your insult aside.
You're not arguing from a standpoint of an unbiased approach. This is literally you going "Fuck this is not what I wanted." and justifying it based on a highly irrelevant point to this specific situation: that is that the katana is the most commonly-used weapon in manga. For very good reason. Also, historically speaking the naginata was considered a woman's weapon for quite some time, used by female castle defenders while the men went out to battle with their swords. (And spears and longbows, but guess which saw the most action and glory?) So there's likely some less obvious connotations involved that make the choice for the writer. The katana has a very storied history, and a lot of national, personal, and familial pride went into them, and it's one of the things that is most visually recognized as denoting something as "very Japanese".
In this case, the Japanese man who may or may not fully remember his past life, picked up a Japanese weapon that is VERY obviously Japanese. We'll see what significance this has instead of bitching that he's not using a Western weapon for no reason (congratulations to the mangaka for not making the chuuni-ass realist another westaboo) or in your case, complaining he's not using a woman's weapon when he's supposed to be some manly faux-alpha harem bad boy or a number of other weapons that are also not particularly suitable for what we know of this character so far.