What's the visual novel that the quote is from?
The visual novel is called Burning Bright in the Forests of the Night.
It was first released as an extra with the first pressing of the 6th Japanese DVD/Bluray volumes on December 18th, 2013, along with other two visual novels: A Choice with no Regrets, and Wall Sina, Goodbye. It has been re-released on the recent Season 1 DVD/Bluray Boxsets.
The whole visual novel is basically Levi’s thoughts about Eren during the events of Season 1. We get to see his point of view during the most relevant moments Eren and Levi share.
It really is great because we get into Levi’s head and know what he thinks about Eren. Here’s a resume with the most important points:
Levi calls Eren a monster, but a monster “created so perfectly it inspires awe, and fear”.
When Levi realizes Eren has transformed to fight the female titan in the forest, and he might be in danger, Levi’s heart is described as “cold as ice”, and he “flies like a released arrow”.
Levi beating Eren in the trial is described as “an extreme, cunning performance. But a necessary one”. He also thinks how they’re forced to make hard decisions all the time, and beating Eren is his last resort. “Ahh, what a load of bullshit. But there’s no choice. A soldier’s profession is to wield violence”. I personally love this part because it really proves that Levi did not want to hurt Eren, and was just doing it because he had no other choice.
Levi knows about Eren’s past, about his training time, and even about his childhood. It is said that “violence and any other form of pain will never break the boy’s spirit”. Not even getting a beating is going to destroy him, on the contrary, it will make him stronger.
The most important part from the whole visual novel is probably this one:
“Humanity’s Strongest understands Eren Jaeger very well.
Eren Jaeger is indeed feared as a monster.
Eren Jaeger is undoubtedly a monster.
But all these people have gotten it wrong from the start.
Eren Jaeger is not a monster because he has the power to shift into a titan”.
When Levi calls Eren a monster, he isn’t talking about his shifting power. He is talking about his strenght and will. He is a monster because he is capable of enduring so much and still keep fighting. When Levi calls Eren a monster, it’s not something with bad connotations, it’s actually almost a compliment, as he speaks of him with admiration.
Eren is a monster “not because of the titan power, but because his soul itself is a monster”. And nothing can’t control this monster, “not even love” (ﾟωﾟ;).
Levi wants to protect Eren, and calls him a “monster with perfect, fearsome beauty”. ( I assume this is the quote you saw in the first place).
When Levi rescues Eren, he yells his name. He “looks at the boy he’s holding" and wonders if he put too much weight on his shoulders or confused him by making him decide.
The visual novel ends with the poem The Tyger by William Blake, which gives name to the visual novel.
I personally recommend this visual novel, it’s short but I think it’s very important to understand Eren and Levi’s relationship a bit better.