Kael's A3 can apply up to 4 strong poisons and his A1 can apply a small poison.  I agree...Frozen Banshee is great...that's why I have her 6 starred and on my CB team along with Kael.  Everyone else on my CB team is strictly there to keep them two alive.

You are underestimating Kael.

Peydma is very good against the clanboss. It's not just my opinion, a lot of people that are very deep in this game, like Ayumi or Hell Hades, put her in high tier vs. CB (Ayumi: 5 stars vs. CB for Peydma and Jareg, Hell Hades: 4,5 stars for Peydma, 5 stars for Jareg). 

Peydma has 100% debuffchance on her A1, what makes her so good in decreasing Atk, and she can steal the Atk-Buff all Non-Void-Clanbosses have. 

You are at an early point of the game right now. Many ways will work. I recomend to 6* Peydma next. If you choose Apothecary or Jareg or another top tier champion instead, you will achieve your goals as well. In the end it's your account, so your decision.

I think at least we all agree Apothecary is your next choice.

Neither Kael (by far best choice for starter )or Athel are late/end-game viable.  I don't even consider them mid-game worthy beyond 12.3 farmers.

I think I might vote for Jareg 3rd, both he and Peydama are 60 worthy but if u could get lucky and get the cooldown on Jareg with only 1 book.  Jareg easier to gear for a new player.  Epic/Lego books are the limiting factor, now if you are spending $$$ that changes everything.  But be wary of ratings on champs, as some require books to go from average to great.  And some are much harder to gear.  Also, ally protect is a very underrated skill, useful from early-game to end-game.