Personally, I think the Kael is okay as he is. Is there room for improvement there? Sure, but it's not going to make a huge difference right now.
My recommendation would be to get Duchess to 6 stars. Give her the absolute best items you can for speed and HP. This alone should make it possible for you to clear Dragon, Ice Golem and Fire Knight up to lvl20, and then you can farm some better gear in general. Even if you say you have crap gear right now, give her the best you can. Speed boots, and HP% gloves, and as many speed/HP% substats as possible on the other items. Chestplate can be either HP% or resist for now.
Once you can farm those levels, your next goal should be to get all items on those listed champs to a minimum of 5 star purple gear or better, with the recommended stats for each type of champ.
If you want advice on what stats are best for those other champs, thats no problem - but I recommend building Duchess first. Prioritize one thing at a time. People aren't messing around when they say she will change the game for you and make things easier.
Aim for 200+ speed and 40k or 50k hp to begin with, that shouldn't be hard. Later on you will want to get those stats much higher, but those amounts are pretty easy to start off with. If you can get her some decent resist too, that will be helpful.
If you have legendary books, use them on Duchess before any other champ until she is done. Even if you don't she's probably still your most useful champ. You can't really lose if the enemy cant kill you.