REX is unlocked by completing the challenge "Power Plant". (Repair the broken robot with an Escape Pod's Fuel Array.)
The challenge requires the player to acquire the Fuel Array item at the back of the escape pod at the beginning of the game, and escort the equipment all the way to Abyssal Depths. Within the area, the player has to make their way to the top portion of the map via the geysers placed around the area, where you will find REX's model as an interactable. Interacting with REX with the fuel cell equipped completes the challenge, unlocking REX as a playable survivor in future runs.
Note: The Fuel Array occupies your equipment slot, but as long as you escort the equipment to stage 4, you can temporarily equip another equipment in prior stages to charge the teleporter and clear bosses without the Array's penalty.
Also, beware that the MUL-T and Acrid survivors are unable to obtain the Fuel Array solo, as they deploy without an escape pod at the beginning of the run.
The Primordial Cube can be particularly effective on REX as you are able to launch it into your Tangling Growth, ensnaring a large number of enemies at once which in-turn massively increases your total healing.
Walking backwards, jumping and using DIRECTIVE: Disperse at a downward angle can launch you quite a long distance in a desirable direction.
Using Seed Barrage resets the attack cooldown of DIRECTIVE: Inject, allowing you to attack much faster than normal.
It is highly recommended that you avoid Transcendence and Wake of Vultures when playing REX as these items can replace your health with Shields, a type of health which cannot be recovered from healing. This can totally cripple REX as his abilities Tangling Growth, Seed Barrage, and Bramble Volley take a percentage of your health, but you are unable to recover any of it by using Tangling Growth, DIRECTIVE: Inject, or Bramble Volley.
It is highly recommended that you avoid Brittle Crown when playing REX as at least one of your four skills will cost health to use, causing you to lose most, if not all, of your money with every use.
Using Bramble Volley on at least 3 enemies can restore you back to full health
Bramble Volley cannot proc on hit items
Razorwire is a great option for REX because the self damage can proc its razor discharge.
Genesis Loop is great for REX because they can manually activate it through self damage.
//--AUTO-TRANSCRIPTION FROM HYDROPONICS BAY OF UES [Redacted]--//
“Status Report on Hydroponics Unit. Serial key begins with 12ea031lsd095a-“
“Yes, 12ea03. Tasked with care for the cabbage, lettuce, and mizuna growths. About 250 plants total. Stable, but losing biomass. I think there are issues with the microgravity. ”
“Keep an eye on the biomass, we might have to scrap 12ea03 and replace it with one of our evergreen Units. Moving on… serial key crp012d054jd0-“
“Crp01 is doing alright. Tasked with corn. About 80 plants total. Lost 6, down to 74.”
“Sustainable, I suppose. What about… serial key rex0ch12d66m-“
“Oh, that Unit. Yeah, it was tasked with care for with one of our cabbage hybrids. To be quite frank… it turned out terrible. I think we started with 80 units, but we’ve lost 79. The hybrid species just isn’t spliced right, I think. There was no way to sustain them.”
“And status report on the Unit?”
“Uh, well… it’s out on the star deck. It managed to hoist the last hybrid onto its platform base, and it’s been walking around the ship ever since, chasing down starlight. ”
“…What? The Unit left Hydroponics?
“So the Unit is disengaging from protocol?”
“Well, yes, but-“
“And instead of terminating the Unit, you let it roam around the ship unsupervised?”
“Just the team, well we just- we just kinda feel bad for the thing, you know. It’s trying really hard to keep that last hybrid alive, and-“
“Go terminate the Unit.”
“Well, the thing is- Rex is actually doing a great job! The hybrid’s biomass-“
“Are you kidding? Terminate the Unit. You have no idea what happens to a robot once it disengages from protocol. You are putting the life of your entire team in danger for the sake of a stray hydroponics unit. This should not even be a discussion.”