Here's some concept art I just realized that I never posted.
For Lefty, being the logical side of the brain, I wanted him to look cold and calculating, but with enough appeal to find likable. So I went with a chrome, space-age robot look, but with an enlarged head and eyes to give him a sense of child-like curiosity. I was watching a lot of Doctor Who when I designed Boomer, so he naturally ended up looking like a steampunk British dandy.
For the live-action segments, I wanted their real-world manifestation, Tom, to be deceptively unassuming, while the romantic interest, Ava, would be more lively and vibrant. I gave the actor playing Tom a grey hoodie and pants, and the actress playing Ava a red coat and teal scarf (which also mirrored Boomer's red pants and teal tie).
For Lefty's room, I wanted a space-age rocket chamber with an LCD screen displaying his visual feed, and a computer displaying other information in wingdings, so that it could look meaningful without me actually having to write anything.
Boomer's room, being more cluttered, required me to get a little deeper into the nitty-gritty of the details. I went for more of a steampunk look to match Boomer's stylistic sensibilities, and a glass container of color-changing smoke to add mood.