Of course, upon general viewing of SNK and its characters, it's clear that they have established Erwin, Levi, and Hange as the leading trio of the Survey Corps. They represent this faction of the military and in interesting ways, mirror the main protagonists of the series.
While there is a lot to be said and appreciated about Levi's relationships with Erwin and Hange, I have come to find that the character who humanizes him is the lovely (albeit forgettable) Petra Ral.
Early in the series, we are introduced to Levi as Humanity's Strongest Soldier. Erwin makes use of his abilities in this sense, and Hange often relies on his strength in combat as well. That is not to say this is the extent of their relationships: we have Levi's iconic "Give up on your dreams and die" scene with Erwin (S3 E16) and the infamous "Maybe we should just live here [away from my responsibilities] together" request from Hange (S4.2 E8). However, something to note with both scenes is that while they give depth to Levi's relationship with both characters, he essentially serves to highlight others. In the former, Levi's response directly challenges Erwin's dream, and allows this commander a defining moment of growth. Similarly in the latter, Levi is used as a means for Hange to express and eventually overcome their fears and insecurities about the deal they've been dealt as the new Commander of the Survey Corps (which at the time was rapidly disbanding under the Jaegerist movement).
In other words, Levi is the "subplot character" to Erwin's and Hange's individual arcs. (According to John Truby, 'The subplot character… provides another opportunity to define the hero through comparison and advance the plot.')
Which begs the question… at what point in the series, if any, is Levi defined as a character in and of himself?
Two prominent scenes from season 1 come to mind, which are namely: 1) The dying soldier scene (S1 E9), and 2) Petra's conversation with Eren in headquarters (S1 E15).
In the first, Levi comforts a dying soldier and vows to carry on their will and exterminate all Titans. When the soldier passes before he can reply to Levi's words, Levi turns to his fellow soldier Petra and asks her if he was heard. Petra provides confirmation, emphasizing the peaceful expression on the soldier's face.
In the second, Petra confronts a gloomy-looking Eren, who has been tasked to clean headquarters along with the rest of the Special Operations Squad (aka the Levi Squad). She specifically points out how Levi is 'not the hero he's expected to be' in the sense that he has a terrible personality, though she does so while smiling almost fondly, as though it doesn't matter what his personality is because they can always put faith in him as their Captain. It seems she wants Eren to understand this - or a notion similar.
It is in both moments that we are able to clearly see Levi beyond being a powerful soldier. Always, he is a threat. When he enters the scene we expect the shift in the dynamic of the battle - we expect him to win. He's a trump card. Erwin's last words to him are an order (S3 E16), and Hange's last words about him is "he's [Armin's] underling now, so really put him to work" (S4.3.1). Levi acknowledges Erwin's and Hange's humanity, bolsters it even, with the conviction of "dedicating your heart". In SNK he is the symbolism, the embodiment, of a soldier. That's all he really ever gets to be. Even his softest moments with - heck, anyone in the series - are meant to deeply reflect on the guilt, the burden, the purpose of getting the job done. ("So… you're telling me… I've spent all this time and energy running around killing people?" (S2 E12) / "Just think, if your hands were still clean... Jean wouldn't be here right now." (S3 E2) / "If we just run away and keep on hiding, what will we have left?" (S4.2 E8))
But for those singular moments in season one, he's more than just the threat. We see him as a human not only with (personality) flaws, but also with dreams and convictions, tied so seamlessly with his comrades' cause that we are reminded painfully, at the end of the series, that it was Levi who carried them all to the end. Throughout the story we see Levi lament fallen soldiers; we are exposed to how much he empathizes with his comrades and their deaths, to the point where it can be said that no one keeps us more aware of the lives that have been lost throughout the show more than Levi himself.
In this manner, Petra was the subplot character to Levi's hero. She gave the audience a (subconscious) glimpse of the Humanity within "Humanity's Strongest" and built the bridge that would lead us to compelling and important revelations about Levi's thoughts and actions as the show progressed. It's Petra whom he finds tending to a dying soldier; Petra whom he asks for confirmation that the soldier heard; Petra who, against all expectation, asks Eren to see past the station, the status, the soldier, to the person himself.
"He's not quite the great, perfect hero society makes him out to be, huh? The real Captain Levi is shorter than you'd expect, temperamental, crude, and unsociable. (…) You thought that because he's skilled, he doesn’t have to follow the rules like everyone else?" (S1 E15)
It's even Petra who, despite her rank, asks Levi to step aside when Eren becomes a half-baked Titan. Here, Levi's robust intuition and split-second decision making skills are shown even away from the battlefield. It's Petra who leads the Special Operations Squad in their apology to Eren (S1E19 "Bite"), who first instills in us (narratively) the notion of trusting your fellow comrades. More specifically, she is who convinces Eren to place his life in their hands. It's this notion that Levi carries with him even until the final arc - "I've saved Eren countless times over - each time, more comrades dying. All because I believed he was the hope of humanity." (S4.1 E13) In the manga (Ch112), it's Petra we see at the forefront of this belief.
"Do you, Eren? Do you find it that hard to trust us?" It's Petra who dies, her words the final say in convincing Eren: "I believe my squad will be victorious." (S1 E21)
"It's like some awful joke," Levi reflects later on, as his comrades' dying hopes and dreams flash by in the canopy of the forest. "What the hell was the hope that we saw? Such bullshit. It's not even funny." (S4.1 E13) "We" here could definitely mean the soldiers who've given their hearts, but the metaphorical representative of this heart is Petra herself... "Eren! Trust us." (S1 E19)
And in the end, it's Petra in the forefront alongside Erwin and Hange, representative of her fellow soldiers, the one (experienced/veteran) Scout we've seen and interacted with in the entire series to have professed the values of hope, of trust, of belief, which is henceforth carried on by Levi himself, his own convictions, his own dreams. They are, in the entire series, the glimpse we get into the Scout Regiment beyond the series' titular character and his comrades in the 104th, and a thorough dive into what makes Levi Humanity's, not simply its strongest.
Her character song, "The Light of Dual Wings", can literally be taken as an allegory of the dreams the Scouts have entrusted to Levi. That's how prominent she is as a Scout; how coded her devotion to Levi is, whether interpreted platonically, romantically, or narratively, as the dedication of hearts.
So, yes, I love them together. I love their scenes, the implications of them narratively, the values Petra professes so effortlessly in the air, washed away by the higher tides of the Female Titan arc. I love that it's still Petra at the forefront, in all of Levi's reflections moving forward, because she is our first glimpse into Levi's character, the real him.
In the end this is just a ship post struggling to keep from delving too much into the symbolism of Levi and the Scouts (how Levi is the face of the Scouts more so than Erwin himself, really), the truest depiction of humanity's collective fight for freedom in this entire series. In his early days, Eren wanted to be a Scout, after all. It is Levi and Petra who push him forward into 'that hell' - for better and worse, respectively.
