@Animaeline
The notion that "katakana has to be romanized into foreign word" is an oversimplification, foreign word could be used for guide to romanize those katakana written word but it's just a guide, moreover on naming.
We don't have to look far for this as even this series author is using katakana for his Japanesse name, and it didn't romanized as "You" (at least when I write this, who knows what happened later).
also, the katakana form for economy is
エコノミー, ecology is
エコロジー
I'm not saying that using Eco is wrong per se, there's even a formal usage of エーコ for Eco in Umberto Eco. I was saying that pronunciation wise in my opinion it's closer to Eiko (eh-ee-ko) than Echo (ee-kow).
If it's meant for Echo then it should be written with エコーas that's how most katakana form for Echo is, エコー not エーコ. Even the entry for greek mythology's
Echo in japanesse wikipedia which is
エーコー have a clear notice on it's first line that her name usually written with エコー.
However, using echo for her name is not wrong either, for example to emphasize that she could strengthen magic by "echoing" it - if that's how her ability is. I found it's strange, but if that's how it is, then that's how it is.
If we're going to dwell on this romanization conundrum, there's no end of it except ask the author to give official romanization. Even on name which is clearly come from western loan word could be officially written not in it's actual word. Take what happened on
Kishuku Gakkou no Juliet, you could shout that it should be Romeo as that's how everyone around the world spell romeo name, but the author
explicitly write it with Romio. At that point whether the romanization is strange, you think it's wrong, it sounds not for the right gender, or whatever else whoever wants to complain about, there's nothing you could do about that unless changing the author's mind.