First time creator need opinion and ideas for roll system.

I have yet to name my developing game but, it is starting to come together. My problem right now is finding a d6 or d10 system that could work with my characters and their development. I have my setting on a new planet. The planet Has 2 suns so there is never a time with out day light. Here's the catch though, the world is a GIANT forest. Basically, each tree is the size of a continent where colonies and villages are able to survive on branches. I have it set to where there are many different trees creating a world map but each tree has different sections (branches) where if you reach ground level it is pitch black and vice versa. Now, as far as character development goes, the player is a humanoid character who is born with a spirit companion (weather it be one of the four elements or a turtle depends on the given companion choice, though some companions like a turtle would be great for tanks while an owl would be good for dps) each player has his or her own set of skills and abilities and each companion has their own as well. Through out the game as you evolve into a stronger character you begin to take skills, traits and features of your companion (a hardened shell like skin or creating webs or even causing earthquakes depending on your level and what abilities your select companion has to offer). Now my big problem is what kind of system would help level up the humanoid and their companions at a realistic rate together through combat and every day travel? I have ideas about battle sequence but am not sure if it would work properly, because both the companions have their own skills and abilities as well as the humanoids (they both will be able to give and receive damage as well as cast spells if able to). If anyone has any helpful ideas I would much appreciate it. I will be working on kinks and loopholes through trial and error for a while.