mighij

Look at their portrait, there is a small golden plus to lvl them up to lvl5

I went with Hylea, partially because I had sworn to her that I would restore the souls and because she didn't mind I did not fulfill her task like she wanted too.

Dwarf Berserker, Slave (and just finished first playhrough)

True. Did use them once though. Beefing up my party with 6 plates of food for the final boss fight helped me defeat him. Lost 2 times before that but +60 endurance for each character helped a lot. For normal gameplay it's too much hassle. If food bonus lasted unly you're fatigued or downed they would be a lot more useful. They would have to limit the food bonus to 1 only though.

You normally make more money then your hirelings upkeep but you'll never earn back the cost of the upgrades. The most convenient upgrade is the Botanic Garden. Together with the Curioso shop and the Warden it's the only one you actually want. The Inn was a bit dissapointing. If it gave you all resting bonusses instead of one it would be worth it, sometimes, but as it is there are much better inns in the world. Some even increase you're stats by 6, the best you can get in your own inn is +1stat and +1skill. As Varana said, it was a stretch goal and it more or less, serves it's purpose.

One your characters portrait (on the bottom of the hud) there is a small yellow plus: click that to lvl