So...are the characters human or robots?.

Don't ruin an explanation that makes sense! Transforming might be their secondary function, like Ivis summoning her rapier and slightly changing her shoulder in the process.
And Tesla's robot parts might also become human via those cells. But we wont see it until Tesla promo, a very unlikely thing.

Seraph Crim's cells generate light, but only after promotion.

talk about bringing your own flashlight...