They're all decent choices.

Personally, I've never taken my warmaiden or fellhound past 50, but there's a good argument in favor of maxxing them out.

Orn can solo dragon and ice golem 20 in a good regen set, so I would probably do him, then you can grind those dungeons and bring chickens to graze.

IIrc the stats he needs are roughly 70k hp, 3k def, 220 spd and 300 res

Without context the easy answer is "nobody". Full roster. Go.

@kramaswamy.kr Wait for better options then? Or just pick one?

He's saying show us everyone you have before he gives advice which to work on next

@kramaswamy.kr @MooredRat What do you think? On one else on the roster is worth mentioning.

It really does depend on where you are trying to advance next, but High Khatun, Thylessia, Diana Gloompiercer, Orn & Duedan are all very strong epic toons that you will help you advance across many areas of the game.

Fellhound is one of the fastest campaign farmers in the game.  Depending on how long your Kael takes to do 12-3 or 12-6, that might be helpful.

Apothecary & Warmaiden are always strong choices for Rares, but if you build Thylessia, you won't have much use for Warmaiden outside of faction wars. 

Thanks all. Went with Orn as I feel he works well with Kael with the poison sensitivity. Dungeons are my focus at the moment. Cheers

At this point all are going to help you but Maus Mage next for survivability if you don't get a reviver/healer like Ursala or Drath in one of your shard pulls before then.