The following is just my opinion, and you aren't required to take any of my advice.
I didn't really understand what was going on. I don't know if this passage was from a longer story, but it would be helpful if you gave the reader more back story. Perhaps more of a setting, and more of a relationship between the characters. The was even more confusing. Is the brother alive? That is what I got out of it. If you are going to have that scene at the end then you need to have a foundation to build it upon. As far as your audience knows, the twin brother is dead. Suddenly, at the end, we get a scene between who I assume is the main character and the twin. You need to establish the relationship in the beginning so the reader can understand Justicar's need for vengeance, and so they can truly be gripped by the ending.
Yuor characters didn't have much depth to them. First off, we don't really know anything about the characters but the basics. People have layers, pasts, stories, memories. This is what makes us human. If you want to make your characters more realistic give them theses things. You don't have to write them outright, but put them there. For instance, you don't have to write Alex's dad was an alcoholic. You can give Alex a strong aversion to alcohol as result of his past.
“Good boy. Now come on, let’s go drop you off for lock-up.”
This wasn't improper grammar, but it didn't sound right. It was a bit awkward and I had to read it a couple times to make sense of it.T
“Justicar dropped off the ledge to land just in front of him, fist in the pavement. The hero stood, ice eyes boring into the villain.”
Was this supposed to be fist on the pavement? If not ignore.T
“No, you s***-stain, you killed my twin brother. Lucky for me and not for you, we have the same powers and he had a back-up suit.”
Back-up should be backup.
“I’ll tell them. I’ll tell every one of them that you aren’t Justicar.”
This depends, if you meant "tell every one of those cops", then yes this sentence is correct. If you meant "I'm telling everyone in the city", then delete the space between every and one.
"She adjusted her helmet, getting the chin strap back into it’s proper"
You confused a contraction and a possessive. I mess up on this all the time. It's should be its. This is because it's is a shortened version of it is.
“What should we put down as his crime?”
You used an incorrect preposition. Change as to for.
"The Commissioner stepped forward and Justicar looked down, hands tight fisted."
You are missing a hyphen between tight and fisted.
This story has a lot of potential if you smooth out the kinks. Just keep on writing!/b}