The reason that the standard answer is Kael is that you need a reliable campaign farmer, and Kael is easy to build for that and he does an admirable job in early dungeons and clan boss.  

Monkey can definitely farm campaign for you, but he is not that great for dungeon progression or clan boss.  

You're gonna want to 6 Star Kael because of a shortage of Books. Uugo is great for almost all content but she takes a TON of books and I don't know how many epic tomes you'll have in the next few weeks. Kael tho, you get lots of rare books from the challenges and whatnot.

Kael also puts out a lot of poisons on Demon Lord so he's a great fit for that role and 6 star means more hp/def and damage.

If you plan on spending money and there's a deal with Legendary Tomes for $4-5 each or Epic Tomes for $1-2 each, then *maybe* uugo. 

All that said, the two best ways to progress your account right now are Dragon Gear Farming and Clan Boss. Top Chest on Normal Demon Lord possibly gets you Ancient/Void shards and Epic Tomes. 

With the Apprentice Pass it's even easier to book them out now.  Those challenges basically walk you thru maxing out your starter.