What the Japanese text adds to Miquella's characterization/motivations
The oft mischaracterized Miquella can be seen more accurately in the Japanese version of the text. A lot of this will be paraphrased from this article: https://medium.com/@Mirko_LaMi/shadow-of-the-erdtree-lost-in-translation-eng-0e7701104786
For example: his main goal was to remove causality from the world.
while the English adaptation of the rune fragment talks about Miquella wanting to be reborn as a god and “to bury the original sin,” the Japanese text tells us that Miquella wanted “to become a new god who would embrace everything, beyond the causality that has persisted since the beginning” — and no mention is made of any “original sin”.
And the Japanese text and retranslation:
ミケラは、影に隠された塔に向かった
Miquella headed towards the tower concealed in the Shadow,
その黄金の身体も、力も、宿命も 全てを棄てて
forsaking everything: his golden body, his power and his fate.
はじまりから続く因果を超えて
To become a new god who would embrace everything,
全てを抱く、新しい神になるために
beyond the causality that has persisted since the beginning.
the “causality” in Miquella’s Great Rune description is the same principle of Golden Order Fundamentalism
万物を関係性の連環となす、意味間の引力である
It is the force of attraction between meanings, which unites everything into a single chain of relationships.
Miquella ascended without the Elden Ring. He was abandoning not just his flesh, but everything gold the entire world was based on: a cycle of suffering. He never intended to use the Elden Ring at all.
Characterization-wise, he is more a good-natured and innocent child with the power to change things, rather than abusive or obsessive as some characterize him as online.
幼き日、ミケラはラダーンに王を見た
At a tender age, Miquella saw a King in Radahn.
脆弱な自分たちにはない、強さを
In his strength, that they lacked since they were frail,
そして優しさを
but in his kindness too.
だからミケラは純真に願った
That’s why Miquella innocently pleaded:
私の王に、なってください
“Please, become my king”.
This is taken from the Japanese text of the "Remembrance of a God and a Lord", and was re-translated to be a bit more literal. The terminology described in Miquella's "request" to Radahn feels a bit different in Japanese. To quote the linked article
The term used in the original description, 純真 (junshin), can be translated as “pure”, “innocent”, “naive”, and “unspoilt”, an adjective that underscores how his request was not only sincere but also derived from a purity and naivety that only a child could possess.
In other words, the remembrance of a god and a lord is intended to highlight Miquella's purity, not Machiavellianism. Same with the ending cutscene:
兄様
“Lord brother…
私は必ず、神になります
I will not fail, I will become a god.
ですから、私たちが約束を守れたら
So, if we keep our promise,
私の王になってください
Please, become my king.
…世界を、優しくしたいのです
I want to make the world kind…”
Rather than say "make the world a gentler place" he says he wants to make the world kind, as if it had never been kind from its inception. Indeed, from all the suffering he's seen, how could that not be the case? He tries to destroy the very foundation of the world - including those that use the Elden ring, the same foundations that gave rise to his suffering, his sisters', and the genocides and oppression of the downtrodden that he came here to save as well as those who live in the Haligtree.
It is also worth noting that the Japanese retains similar language to the English in terms of the nature of the vow: "if we keep our promise" referring to something outside of Miquella becoming a god, and Radahn becoming king, as well as a third party indicated by "we". "So, (aside from becoming god) If we keep our promise (something unrelated to Radahn being king)" "THEN please, become my king". It's conditional upon upholding something else. There was a demand Radahn made that Miquella had to uphold. A condition. And Miquella would do anything to appease him, so he promised him he would do it. However, he did ask that Radahn promise to be his king in his age of compassion in return.
The boss fight monologue gives insight into part of what Miquella's promise to Radahn is:
そして約束は果たされ、強き魂が還ってくる
And the promise is/will be fulfilled , the strong soul returns
Indicating that Miquella's promise to Radahn involved bringing back his soul after he died in battle. It suggests that Miquella wanted to do everything on Radahn's terms, the man who he looked up to - Miquella respected Radahn the most, and the act of him bringing Radahn back was to honor his wish regarding the manner in which he'd become King.
Miquella's true motivations are also highlighted by his boss fight dialogue.
すべての生命と、すべての魂に
To all the souls and living beings,
これよりは楽園の世紀
herein comes the Age of Paradise/Eden.
優しき理、千年の旅
The Law of Kindness, a thousand year voyage.
愛だけを思うがよい
From then on, only love shall come for you.
Nothing and no one will be condemned or rejected
P.S: this dialogue is cut content.
…褪せ人よ
Fading one…
旧律の王たる者よ
King of the old Law.
貴方が罪を知り、世界を憂うのなら
If you know sin, if you grieve for this world,
我らに道を譲り給え
then I ask you to give way to us
ミケラと
To me, Miquella,
ラダーン、我が約束の王よ
and Radahn, my promised king.