You do it via overt and explicit character development. I.e. monologues, dialogue, interactions, stuff like that. I fucking hate this. This has never worked for Samus. It didn’t work in Other M, it didn’t work in the few odd instances in Prime 3, and no, in spite of how much it might be a fan favorite, it didn’t work in Fusion either. In fact I’ll argue that all the excesses and problematic portions of Samus’ characterization in Other M were ultimately the logical end point of character tendencies already introduced in Fusion. It’s especially notable that Fusion’s most memorable and resonant character moments for Samus aren’t where she’s monologuing about Adam or the animals she rescued, they’re entirely gameplay and mechanics driven ones, such as when she goes off script from ADAM’s orders, or her final encounter with SA-X/Omega Metroid. Perhaps under better writers who are capable of handling overt storytelling well, I could see Samus being done justice, but not only am I not sure Nintendo, for all their considerable talents as game makers, have that writing skill, I’m not sure there’s anyone in the games industry I would trust to pull something like that off to any satisfactory degree. Because the best we’ll get out of someone trying to characterize Samus by giving her proper words is something like Aloy, which, fuck no. The worst? Other M. Which, FUCK NO.
I don't think you're giving Fusion Samus enough credit. I find that depiction of the character to be a particularly strong one, for several reasons.
This is the game where Samus is the ultimate underdog, and the weakest she's ever been. She nearly dies, right at the beginning, and almost immediately after that, she's already being sent on another mission. Despite hardly having any time to become acclimated to her new physiology, Samus steadfastly navigates the horrors of B.S.L. in the way that only she could. She ruminates on, and draws from her past experiences, clinging to her humanity, despite her only ally being a computer telling her where to go (she remarks on the coldness of the AI, even), and despite not quite feeling like herself, physically. She feels fear, yes (as anyone would, when going up against what is essentially John Carpenter's The Thing), but despite her weakened form, she's able to hold onto who she is, maintain her ideals (ultimately convincing the computerized Adam to live up to the CO she previously admired), and rebel against a bioterrorist cell of the Galactic Federation, consequences be damned... because she's one of the few (besides the Chozo) who know the true threat the Metroids and the X pose to the galaxy. She's lived that experience.
I'd like to share my favorite Samus moment from Fusion. It takes place soon after the secret Metroid lab gets jettisoned off B.S.L., and just before she sets the station on a collision course with SR388.
She's outraged, right? Outaged, as anyone in her position would be, that she's gone through hell battling the Metroids and X, and yet again, a group of evil morons want to use them as tools for galactic dominance and control. After a game of being railroaded and told exactly where to go, Samus refuses to be a pawn for the Federation any longer, and makes her own decision, for the greater good, with no regard for what will happen to her.
As soon as the Federation ships land, the X will overwhelm them and absorb the knowledge and power of their occupants. The X are just waiting for the hunt to begin, and then they'll spread across the universe. Galactic civilization will end. The X hunger for form, knowledge, and power. They mimic these perfectly. But they cannot copy the soul. They're single-minded, instinctively seeking to increase in number. They're a plague, and the Federation underestimates their threat. The X must not leave here. I must destroy them all before the Federation arrives. This station has a self-destruct mechanism. I must use it to destroy the X here and on the planet. I must send them to oblivion. Them, the station, and myself, if I have to.
Now, if that monologue isn't the perfect encapsulation of threat assessment, selfless heroism, and full-on conviction, I don't know what is. This is the Samus I'd call badass, not her doing some cool scene from an action movie. Metroid 1-3 were great at giving us a picture of Samus's competence, but Fusion is the first game where we bear witness to the fact that she's not just some space cop doing any dangerous jobs that high authority tells her to. She's battling threats around the galaxy because she wants to keep it safe, even if she has to die for it. That's just who she is, as a person.
During the ending, she knows she's done the right thing, and she's at peace with that (not to mention her downright philosophical reflections after she escapes). Samus Aran was a true maverick in this game, and I loved the hell out of it. Fusion was my first Metroid game, and I'll never forget the strength of Samus's character within it. Yeah, maybe the whole "Samus Aran is now a galactic fugitive!" implication was the result of mistranslation work on the English localization, but that still would've been a hell of a direction to take Metroid 5 in, that's for sure.
As for Other M, yes, that game's characterization of Samus is a complete and utter disaster, but I disagree with it being the logical endpoint of what Fusion started. I'd say there was no "endpoint" for what Fusion established in the Metroid canon, beyond what it already was. The game spoke for itself, stood on its own, and closed its own threads. We didn't need Other M. We didn't need to actually see Adam and Samus's relationship, and if we did, he didn't need to be portrayed as someone that Samus would seek consent for to use
her own damn suit functions. Fusion's endpoint was itself. It's a shame that, at the time, Sakamoto didn't understand that, but he seems to have course corrected, and thank goodness for that.
In closing, am I agreeing, or disagreeing with you, regarding your assertion that Samus is better characterized through things like subtextual actions and body language, as opposed to narrations and internal monologues? Well, it's complicated. I absolutely see where you're coming from. If they can't write Samus well, then they'd better not do so at all, lest we get another dogshit depiction... but at the same time, I truly believe that Samus is a deeply compelling character, and that perhaps some writer in the future will be able to have that shine through again through spoken word, just like it did in Fusion, and just like it does in the games that aren't Other M.
(Sorry for the wall of text. I just love these games, and this character.)