A strong character has two elements:
- Its motivation, as revealed through its choices, and
- Its personality, as revealed through its dialogue and body language.
So let's talk about how those apply to this situation!
You AI's Motivation
What does your AI want? Perhaps it just wants to keep the player from dismantling it. But I suspect it goes deeper than that, you seem like the kind of writer who knows the importance of a clever secret. You can give your AI an incredible amount of deep characterization just by having it make decisions that go against what the player would expect from it, forcing them to reconsider what they think the AI's true motivations are.
A classic video game example is the first encounter with Flowey in Undertale. When he first appears, he is incredibly welcoming and friendly towards the player and offers to give them a tutorial on the game's action system. But when you grab one of his "friendliness pellets," all of a sudden, the mask comes off. He was actually trying to kill you, and the friendliness was an act. All of a sudden, the player, who had accepted that Flowey was a nice guy, has to reconsider everything they know about him. What kind of monster would kill someone as quickly as look at them? That question isn't answered for a long time, and the mystery of Flowey's true motivations becomes one of the driving forces of the story.
Another excellent example, one a lot closer to your plot, is the main antagonist of OneShot. In this game, a young boy wakes up in a strange, dying world and must carry a sacred light bulb to the center of the world to reawaken its sun. But throughout his journey, a strange voice, communicating with him through computers, tempts him to destroy the bulb. When it becomes clear the boy isn't willing to listen to him, the voice then reaches through the game to talk directly to the player - and the claims it makes throw the meaning of the entire journey into question. It turns out that the mysterious voice has a very compelling reason after all for wanting the boy to destroy the sun. Its decision to unapologetically harass the poor protagonist casts it as an unambiguous evil at the start of the story, but its later decision to talk to the player hints at a mysterious, deeper motivation. That deeper motivation drives the rest of the story from that point.
In your case, the player is going to get used to the AI acting to hinder them pretty quickly. But perhaps sometimes, the player is going to attempt to do something that actually lines up with the AI's true motivations, or at least something the AI thinks it can manipulate in its favor, and the player will find the AI clearing the path forward for them. Or maybe when it becomes clear the AI is not going to be able to stop the player, it begins trying other ways to get into the player's head - lying to them, coming clean with a partial version of the truth, giving the player a partial victory to try to convince them to leave what it considers the most important alone. When it starts making these decisions that run counter to what the player expects, you will reveal a greater depth to its motivations, and it will be a much stronger character for it. You don't need any dialogue at all for this to be very impactful.
Your AI's Voice
Voice is picking out a defining personality trait, quirk, or manner of speaking for a character and honing it to the point that we can often identify who's speaking just from the way it's said. If your AI can only speak in pre-recorded phrases, that's honestly a strong voice in and of itself! I don't think it's necessarily a problem. It will make your character stand out in a good way already.
We can go deeper with it if we want to, though. Once you've established a voice, you can make it richer by playing with its nuances and subverting it when the character is feeling overwhelmingly strong emotions. TVTropes calls it Out of Character is Serious Business: When a character drops their usual voice and suddenly begins expressing themselves differently, it's a very clear signal that something very important is happening to them.
Take Flowey's example again. (I get into Undertale spoilers in this paragraph.) He very quickly is established as a sociopathic monster who loves lording how much more powerful he is over the player. His typical voice is on full display during the Omega Flowey boss fight with his deliberately grotesque omega form, his constant taunting of the player, and his goal of just killing the player over and over again forever. But in both of the true endings, we see a very different side of him, ones that make us realize that there's much more going on. In the Asriel boss fight, when Flowey initially transforms into his true form, he still lords it over the player, as we expect. But as the player gradually whittles away his ability to control them, he doesn't react by getting angry and trying to reassert control like he did in the Omega Flowey boss fight. Instead, he gets desperate and starts crying like a little child - which is exactly who he really is at the core. This unusual change of voice shows us, during the final boss battle, that there's more to Flowey than we had heretofore believed. When Flowey is killed during the genocide route, his pathetic groveling briefly shows the same side of him.
Here are a few ideas for how you can play with your AI's voice:
- It can rearrange its programmed voice clips to formulate new sentences. Hearing a familiar phrase suddenly have its meaning inverted because of a cleverly spliced "not" could be quite chilling!
- It can use the same voice clips in situations they weren't intended for, giving them a double-meaning that speaks to the AI struggling to communicate with the limited tools it has. Maybe when we first enter the facility, we're in a nice lobby, and the AI invites us to "Please make yourself at home." Then later, when it's tired of our meddling and traps us in a dangerous room with all the doors locked, it sarcastically repeats itself: "Please make yourself at home."
- If you really want make a point, let the AI say something new, something that wasn't in its programming. If there's something so important for it to say that it finds it worthwhile to dig deep and synthesize new speech, it's got to be a really big damn deal. And it will hint at hidden depths, maybe even a spark of sentience...
- As the AI breaks down, either as the player takes it apart or it becomes too angry to contain itself, you can distort or corrupt its voice clips.
All of these give you ways to just get more mileage out of the voice you've settled on if that's all you want. But they also give you ways to break out of character to signal when we've touched on one of the AI's raw nerves.