UPDATE 2: This is just based on the trailer. The full lyrics have sinced been released, and I translated them in full HERE.

UPDATE: It has been revealed that the name of the singer is Aqua. The official song’s name is “if – hitori omou”, or “One’s thoughts.” The below is still based on the trailer, but when the official song comes out, I’ll update it with the official lyrics.

A user on serenesforest.net kindly pointed out that a Japanese 2ch user uploaded potential lyrics for the song. So I translated them best I could. Please note that the translation is based on their interpretation of what they heard and transcribed, so if any errors arise, that is one possible reason.

I am still working on ironing out the translation. The song in question is during the first half of the trailer below:

The order is Kanji, Romaji, and English.

伝う水筋
その手が開く明日は
光へ手を伸ばす
穢れなき金の剣
まどろみ　想いを断ち切って
伝う静謐
伝承　柔く光散って
闇は去りゆく
暁一人想う

tsutau mizu suji
sono te ga hiraku asu wa
hikari e te o nobasu
kegare naki kin no tsurugi
madoromi omoi o tachigitte
tsutau seihitsu
denshou yawaku hikari chitte
yami wa sariyuku
akatsuki hitori omou

From a vague lineage
Comes a hand that will reach toward a light
Opening a path to tomorrow
The un-corruptable golden sword
Will slash those hopes from their slumber
And lead to prosperity
That soft light will scatter
And chase the darkness away
Like a one man sunrise

—

There are still some things to work on, as　伝う水筋 can mean “along the river bank” or “from a watery lineage(?)” which itself can be taken to mean from a “vague lineage”, or “unknown linage.” In a way this can fit the theme of the game with Kamui being born in Hoshido and raised in Nohr, and so I went with “vague lineage.” The woman is singing on the edge of water though, so perhaps it is a pun?

“Slashing hopes from their slumber” implies awakening the hopes that were once lost, rather than crushing them as some may interpret it as.

With the new name Aqua, the watery image may work better. Also, “hitori omou” is the very last lyric of the song, so it can mean dawning on one’s thoughts rather than being a substitute for a metaphor.