I've been modeling the armor for this suit, and as part of the project also working on undersuit and cape references from the game assets.
For what I've gotten into, I think the best bet would just be a generic dark undersuit (with muscle padding if needed) that then has accenting sewn over the top. It's not that complex of an undersuit for seaming, those lines could be patterned out on someone while thy are wearing the suit and then added to the base through patchwork/piping etc.
I'll attach a ray shot of the exposed areas on the suit, but two things I noticed that could are a bit unique-
1- It looks to me like the bicep armor is underneath the base suit, so I am going to make biceps halves with no real detail to duplicate that look.
2- The undersuit also has both small grommet details, and more complex additions like the bicep/pauldron/knee strapping and the detail plate at the small of the back. Stuff like that can be made in EVA foam and then secured to the base suit as well.
I never found a close real-world version of the undersuit (kind of to be expected as it's a video-game design based off of artist concepts), so it could turn into a bit of work if you want it hyper-accurate.
Here's that image of the exposed undersuit areas (for how I'm designing the suit anyways):
