Okay first thing first, I love how this is spelt outright. I mean, it's been kinda obvious in practice but until now it's never been outright said that Arthur is Shinra's guardian.
And I also love that it's Haumea who said this line. I feel like Haumea-Charon and Shinra-Arthur are both parallel and the polar opposite of each other.
See, both Haumea and Shinra are the images of something 'pure good' (a saint and a hero, respectively). Both are exposed and painted as what amounts to the dark side of humanity. She's got the despair of mankind drilled into her head 24/7 since she was a baby. He's got scapegoated and branded as the devil for things he never did.
To support her, Charon acts as Haumea's punching bag, believing that it's the only way he could help her. He shows her unconditional love, never talking back against her or even protecting himself when she becomes abusive towards him.
In contrast, Arthur fights with Shinra almost all the time. He won't accept Shinra's bullshit (whether or not it's actually bullshit on the latter's part). And, in the manga-only flashback during their academy days, Arthur once crossed the line and genuinely made Shinra upset.
In the end, though their ways differ, both Charon and Arthur become Haumea and Shinra's respective pillar of support. At the very end, Arthur's whacking and yelling at Shinra from space is what pulls him out of furious despair. And Charon never dismissing Haumea and simply asking her to smile once is what pushes her to choose life instead of death. They both succeed in their mission to protect and save their respective protectee.
