Khamari

When you first get Khamari, he will carry your team. He has decent bulk and sustain for the entirety of the Weald Arc. After that, he does tend to fall off a bit. At the start of the late game, when you get Vissarion, his viability goes back up as a tank, but he struggles to find much purpose besides a bit of a semi-tanky support between then and there.

Dreadnought - Khamari is a weird unit. He's kind of tanky.
1. If you run Aya in your party, the Bleed on his personal heal isn't detrimental. Khamari's middling Mastery isn't really needed so if you run the Dimming Ring you can add it to your bulk. Alternatively, if want to keep his Mastery, you can run the Love Ring for 20 extra HP if your bonds are maxed out, the Karmic, Indominitable, Guardian, Hallowed, and Lifebinder.
2. Khamari isn't self sustainable but with a bit of support he can be a main tank in your party. For instance, Vissarion's personal skill will give Khamari A TON of bulk and enemies even in demonic will struggle to take him out. Additionally, he appreciates Repulsor Shield buffs (due to his low speed to get doubled, and Relentless passive ensuring he gets hit) and an occasional heal, but with his Bloodmage passive, the Plague Ring and a good application of Transfusion, he has decent sustain on his own. If you find that enemies are hitting a little too hard, change out the glyphs on Transfusion and maybe give him an occasional heal. 

Tidecaller - Waveseer into Tidecaller is arguably the most S tier of any classes. Anyone who can do it can do it well, Khamari is no exception. However, Khamari can also do a tanky version of Tidecaller with a very similar build to his Dreadnought one - with a few clear distinctions:  less power/defensive bulk but higher speed, and access to 1-2 range. Khamari won't really be one rounding with this build, but bleed will do its thing over time and Khamari has decent sustain with being able to attack bloodied targets at 1-2 range. Additionally, giving him access to Vissarrion and Repulsor Shield will still make him an insanely bulky, potential one rounding threat for the enemy.

Hemomancer - Khamari can enjoy a straight promote if you decide to keep him in his base class. It isn't terrible by any means, it just isn't anything super special for him either. He enjoys the HP growth and that's really about it. Simulacrum is always decent but nothing super special here for him.