It's good that you are asking these questions early. From the looks of it you've managed to avoid a lot of the pitfalls most new players suffer from.
Continue as you're currently going - get Kael to level 60, fully ascended, and then unlock his masteries with gems. That'll let you farm campaign to create food for other champs.
You can probably safely leave Spirithost at level 40, and stop leveling other champs when they hit 40. Your farming runs should consist of Kael + three other champs that are not yet level 40. You should stop ascending Spirithost, and you should not ascend anyone other than Kael for now.
With respect to the actual champs you have - other than Kael, they're all trash, so don't level any of them past 40. The ones with the little spinny blue icon are fusion champs. Take a look at the fusion list, the one you want to focus on is Rhazin Scarhide, so keep any of the champs you get that are necessary for that fusion. Don't level them, just leave them in your vault for now. Use the others as food.
Your on the right track with kael. Get him to six star, full booked and masteries asap.
Spirit host will help early but I wouldn't take her above five stars. You can farm her in campaign to book her if you want.
Also you will want to farm warmaiden and take them to 5 or 6 six stars, and also farm dupes of her to book her out.
Most rares and uncommons will not do anything for you, but starting out armiger a common champion is easy to get and can be good. There are a few rares to look out for as well like frozen banshee, cold heart, coffin smasher, renegade, apothecary, and painkeeper.
You can take someone like sniper to 4 stars to help early and use them as food once you outgrow them.