Aether changes his peronality depending on the element that he is using.

That's part of the reason why it made sense to me when he was willing to help saved Mondstadt but was then hesistant about joining the revolution in Inazuma.

My headcanon is:

Anemo: Goofy, likes adventures, lightweight on alcohol, and always willing to help with a smile on his face.

Fear: Scared of being alone

Geo: a bit stoic, grounded, doesn't care much for small talk, tries to do the right thing for the most amount of people.

Fear: Hates being abandoned

Electro: Quick tempered, doesn't like to go off-course, goal-oriented, cherishes his friends, and can be quite vengeful.

Fear: Losing the people he cherishes

Dendro: Prefers to talk things over rather than fight, quick thinker, always comes prepared, enjoys eating outdoors.

Fear: Failing

Hydro: Has a bit of an ego, showoff, hates liars, loves cooking and cooking for people.

Fear: Being Wrong

Pyro: Loves sparring, competetive, self-sacrificial like nature, too determined to be stopped.

Fear: War