The ranger is the only unit that everybody uses all the time so making it level up would be rewarding in every game.
This way we always feel like doing something useful to improve our team, even if we change our 8 illuvials we still keep some progress.
It doesn't need to scale a lot especially considering how weak it is with the basic suit and weapon, it won't have any significant impact on the outcome of the game 99.9% of the time so it will be more of a psychological trick.
You could even nerf its level 1 stats a little bit down to 400 base HP for instance but it could go up to 800 base HP.
It will mostly have an impact on ranger vs ranger rounds but the strategy of not placing any illuvial in the 1st round to trade HP for info is more advanced and regular players will max out their ranger level relatively quickly anyways.
Also it's our character, it's better if we feel attached to it.
A good way to add stickiness in my opinion, permanent progress is super enjoyable.