Chao is an odd 4*, I have never really been a fan of him due to his below average attack. His skill is offense oriented. But max stats before emblems, he is more of a meat shield.
That said he is in the best class tree to fix his shortcomings with dealing damage. Here is what he looks like with 11 nodes, focused on the attack path;
685 attack, can easily get over 700 (EDIT: when maxed, he’ll have 730 attack which is actually really decent!). I’ve been more impressed with him now. The mana cut is still meh, but the damage actually puts a dent on the enemy rather than just a scratch. Still helps keep bosses and dangerous enemies in raids under control, but now actually packs a better punch. He also picks up decent defense boosts along the way.
It’s funny, he is the only 4* so far for me that has gone from mediocre to actually very decent with emblems.
I have Gretel too, and I do agree to level her before Chao. But I use both when stacking yellow myself on offense. Just requires coordinating their skills well.