Chao specializes in offensive teams, specifically raid and war offensive teams. Gretel specializes in event quests and titans. I like to choose chao when I need a little more survivability vs. another mana controller (I also have hansel).
Gretel’s mana control is fantastic, keeping the most dangerous specials at bay, but she hits like a kitten and her average mana in a pvp battle means it’s easier for her to be killed before charging up.
Chao has fast mana and hits much harder, often taking down other 4* heroes that are at half health. He synergizes very well with hansel, giving his buddy hansel a little extra time to recharge and target a second opponent. I often target chao at an opponent with half health and full mana. When he doesn’t take them completely down they still can’t fire and it gives me a chance to finish them with tiles.
Hansel was my first green 4* and I absolutely love him. I don’t know how i’ll be able to choose between him and lianna when I finally get that last tonic.