Soooo a bunch of people already chimed in with really good advice. I can't say I'll add anything special, but it doesn't hurt to gather viewpoints. I do agree that your writing feels young, it has the awkwardness of a teen/tween drama, or something like Goosebumps/Are You Afraid of The Dark?. Some part of the dialogue are a little hand-holdy, like you expect the audience to not comprehend certain things, if that makes sense.
I do have my own version of how I would recreate some of the dialogue. This isn't so much a suggestion as it is just me showing how I may do it, if your current writing style is most comfortable for you I'd say to keep doing it, your story is free to read afterall. Plus, I'm not your target audience nor someone who writes a whole lot, so it's not my place to tell you what to do.
Spoiler for length
Orion looks at him with concern.
Orion: Who was that? The tall woman you saw.
Null: Cassandra. Caretaker of the Arcanum soldiers, and the Arcanum prophet.
--
His face is more stern now.
Orion: Why did you surrender?
Null: You don't fight Cassandra. She beats you down. Besides, I have no right to attack her.
Orion: Why not?
--
He looks down wistfully.
Null: She's my superior in every respect, always has been and always will be. I deserve none of the kindness she grants me.
Orion: You were scared of her.
Null: I never said she gave a lot of it. But always more than I deserve.
--
Orion makes an uncomfortable face at that.
Null: What? You worry about me?
Orion: No. Never.
Orion: Who was that tall woman?
Null: Oh, Cassandra. She's the Arcanum prophet, and caretaker for our militia.
Orion: Is that why you obeyed her?
Null: I would never win if I attacked her, also I have no desire to fight.
Orion: Why so?
Null: She is my respected superior, and the kindness she grants me is already more than I deserve.
Orion: You looked to be scared of her.
Null: Cassandra is a stern figure, she is only gentle when necessary. Still, it's more than what I person like deserves.
Null: What? Is that odd to you?
Orion: No... It's nothing.
I don't know the context to this, so I could be incorrect in my reinterpretation. I did find repeating Arcanum unnecessary since I imagined she wouldn't be the prophet and caretaker anywhere else, if you've already mentioned them being in this location prior you don't even have to mention it in the dialogue since your readers should the the literacy skills to figure that out on their own. The response you had to BoomerZ's suggestion was interesting. You mentioned Orion only being interested in who Cassandra was and if she's abusing this child (19 is still a child as far as brain development goes.), but asking how she's a hardass could help answer those questions. Null would logically share some info on who she is, and what she does to make her a "hardass", which could enlighten him some on her abusive nature.
Electra is standing there awkwardly in a leather jacket over a sleeveless shirt. Without her helmet you can see her dark skin, her deep brown eyes, and her long cornrows.
Electra: Hello. Is this the house of Penthasilea Valiant?
—
Silea: Electra!
Silea giddily bum rushes her to hug her tight, and Electra reacts to this with awkward confusion and a bit of blushing.
Electra: The answer is yes then?
Silea: Yep!
—
She pulls back and sees the bike has folded into a luggage case, and her eyes go wide and glittery in excitement, and Electra looks away blankly.
Silea: Whoa you really made that?! You're literally the coolest person who ever lived.
Electra: I am almost certain that is hyperbole.
—
She turns to look Electra in the eyes intensely with her hands on Electra’s very surprised shoulders.
Silea: Hyperbole is a made up concept invented by pussies who won't own up to their words.
—
There is a pause and Electra reacts to that phrase in a way she doesn't understand, and Silea tilts her head.
—
Electra looks out into the doorway curiously, a sweat bead on her forehead, and Silea looks out with a focused but unserious look.
Electra: Where do I put my bags?
Silea: I have no idea. We'll have to find space inside.
Electra: Hello? Is this the home of a Penthasilea Valiant?
Silea: AH! Electra!
Electra: I'm guessing that's a 'Yes'?
Silea: YES!
Silea: Amazing! You made this?! You're seriously the coolest person ever!
Electra: No need to embellish.
Silea: Embellishment is for pussies, I'm being sincere!
Electra: So, where should I put my bags?
Silea: Oh shoot! We'll have to find a space for them outside until that's sorted out.
Silea's dialogue seemed intentionally immature, so I tried to keep it that way while also making her come off a little older. The bit about hyperboles being for pussies who can't own up to their words was a little much, very "Feminism Girl Boss" and felt like she was reading a script, it didn't read naturally at all.
Then she bumped into Orion walking the other way.
Silea: Whoa! Orion. I thought you were still crying.
Orion: I was. I just… After everything that happened… I just thought we should talk about it.
—
Silea: I was actually here to talk to you about that.
Orion: Oh. You go first then...
—
Silea: Okay, so... How are you feeling right now?
Orion: Shitty. How about you?
Silea: (sighs) same, honestly.
—
Silea: Do you want to talk about it?
Orion: I think I would, actually. But... What's there to say?
Silea: A lot of things. How are you holding up after all that?
Here's another old one, which is actually pretty outdated.
He leaps off of it, lands in a roll, and spins around dramatically to face Silea as she punches a wall.
Null: What… are you doing?
Silea: I'm training.
Null: By playing bloody knuckles with drywall?
—
He comes closer.
Silea: That's how a valiant does it.
Null: What's being a blueblood have to do with breaking your fists? Orion does it too for some reason.
Silea: You wouldn't get it.
—
Null: Shouldn't you be training with your staff?
Silea: My staff isn't going to stop an army.
Silea: Orion! Are you done crying?
Orion: Yes. I just... I think we should talk about everything that just happened.
Silea: I was actually on my was over to do that!
Silea: So, how are you doing right now?
Orion: Shit, I feel like trash. How about you?
Silea: -Sigh- Yeah, same. Pretty much.
Orion: Yeah...
Silea: Would you... like to talk about it? You know, if you're comfortable.
Orion: I... think I would, actually. I just don't know where to start.
Silea: We can start from the first thing that comes to your mind. How about that?
I changed up the last bit since I didn't think there was a need to ask how Orion was holding up after already asking how he's feeling, it was pretty much the same question worded differently, and changing that last part seemed more productive to the story.
I didn't do that last two dialogue sequences since I'm short on time, but again, I just wanted to share how I would go about doing these scenes. I saw you talk about wanting to show the character's trauma, and it's cool you're interested in depicting that topic, but personally the dialogue was too stilted for me to really pick up on it. Everyone has already made really good suggestions for this, I would add to do a lot of psychology research on the types of trauma you're depicting. Follow therapists discussing that stuff on Youtube or TikTok, follow survivors talking about their experiences, those are really great starting points for understanding the human aspect of trauma. Also, this is a weird one, but I like watching reaction vids where people watch short films or music videos depicting trauma or mental illness. Especially reactions from therapists and survivors, they always have their own unique responses and perspectives on the videos their watching that may be helpful.
Again, I'm not trying to tell you how to write or what to do, I was sharing my viewpoint. Do what makes you happy! At the end of the day this is your story, and you're sharing it for free.