The title is explanatory!

I want to keep going with my quests, but the only Sylestis that I have is my level and therefore I'm scared he'll be stronger and I can't control him! ;-; but I'm stuck at level 16...

As you level up with your starter (I'm assuming you're using your starter in your party), your player level will level up with it. ^^

Edit: I remember there being some sort of way to auto level to 60, but it's better to progress through the story and get used to the mechanics. c:

Don't worry about it!
In Sylestia, your level matches to the highest level that you've raised a sylesti to, and if you obtain a sylesti that's got a level higher than yours, you merely cannot put it into your party.
So if you raise your Vexityx to level 17, your own level will reach 17, and if you get another level 17 sylesti and raise that one to level 18, you'll also raise to level 18.

I'm not sure how things like having a higher levelled pet and then raising its level via missions/EXP orbs affects your level, but it may just cause your level to jump up to their new level?

I think you're talking about how you can buy a high level pet from someone and then send them on missions. When they earn enough xp, they'll level up, and so will your player. I accidentally jumped up to 60, thanks to my mission pets. xD I really should be at level 55.

Thanks, everyone! Now I know xD

Just the old noob :3