It can be observed that while Aether is reserved about himself, he is also not used to being alone. He travelled with his sister for a very long time, presumably for many thousands of years, but it seems to be the first time they are separated. The moment he meets Paimon, she becomes his permanent companion and he easily tells her about being from another world and that he searches for his sister. It's important to note that he does not confess to being an outlander to anyone else, even when directly confronted about this. For some reason, he puts a lot of trust in Paimon and their connection only grows as time progresses. It is best shown when Aether and Paimon are "separated" (Aether actually has a long, realistic dream) during one of encounters with Dain. Once Aether is reunited with Paimon, he hugs her and admits that he missed her, much to her confusion.

Aether's relationship with Paimon is very sibling-like. He often teases her and treats her like she's too innocent to understand some things, but at the same time allows her to speak on his behalf. During their banter, it's easy to see that they get along well and Aether seems to enjoy entertaining Paimon's ideas and either poking holes in them or joking. Their strong friendship is very obvious, especially in the cutscenes. Whenever some danger appears, Aether steps ahead, allowing Paimon to hide behind him and whenever Aether is hurt, Paimon is always beside him, worrying about him or making attempts to get him to safety.

Despite that, there are still things the traveler does not share with Paimon. At some point Lumine shows up and tells Aether that it's not the right time for them to be reunited, causing a great heartache for the traveler. Up until this point he wished desperately to see his sister again, so for her to not answer any questions and leave him alone again stuns Aether. When Paimon tells him to cheer up, he denies being sad. It is likely he does this both for himself and for Paimon's sake, as he is usually seen trying to find solutions from difficult situations as well as trying to look at the positive side of things.

Sadly, this kind of behaviour is taking toll on him. He never allows himself to properly vent or rest, always pushing himself. Multiple characters offer him a shoulder to rest on (Venti) or just that they will listen (Kazuha and Thoma) but he rarely ever takes anyone up on that offer.

When it comes to his surprising meeting with his sister, it causes a change in Aether's goal. Since Lumine left him with little to no explanation, Aether desperately tries not only to find her again, but also to understand her goals and the reason why she sides with the Abyss Order. As far as Aether is aware, the Abyss Order does only harm to the people of Teyvat and thus, the traveler can't imagine his sister willingly allying herself with someone like them. The situation is further complicated when Aether finds out that he is the only recent "Descender" (as people from outside Teyvat are called in Fatui reports) and his sister is not considered as one. This makes him only more confused, but again he chooses to look at the bright side of things.

Despite the optimism, Aether definitely feels some sort of loneliness. It is shown especially well in his character story, that tells of how Lumine is his only kin and that no world could ever accept such alien beings as them. As such Aether firmly believes that his "home" can only exist if his sister is by his side. At the same time, it appears that Teyvat grows very dear to the traveler. During one of the events, when asked where they're from, Paimon responds "we're from Mondstadt" and Aether never corrects her. In conversation with Kaveh, Aether even asks the architect if he could design a home for him and his sister once she is found, indicating that he's hoping Teyvat could become a "home" the twins desperately searched for.

Another thing is that despite his general politeness and empathy he shows towards people, Aether isn't exactly a type of person who would do as told all the time. He isn't afraid to call someone out or even interrupt court proceedings, as he did in Fontaine. Standing in face to face with a god won't faze him either, as shown during the very first moment Aether and Lumine appear in the game. As I mentioned before, this can land him in some really nasty situations. For instance when Thoma is arrested and his Vision is supposed to be taken by Raiden Shogun, Aether rushes to fight the god even though the fight seems impossible. Eventually, he is defeated by Raiden and the only reason he manages to survive is Thoma, who uses the commotion to free himself and distract Raiden. He then grabs Aether and they both run away from the scene. There are many more instances when Aether rushes into a fight, but has to be saved himself. He is unable to just stand and watch, but that doesn't always work in his favour.

When it comes to regrets, there are two significant moments that left Aether in anguish. The first happened when Teppei fell victim to Delusions. The second one was when Signora was defeated in a duel with Aether, then she was killed by Raiden. Both those deaths imprinted themselves on the traveler. First one was a death of a friend, making Aether feel both useless and angry that he couldn't do anything to help Teppei. The other was death of an enemy, but at the same time it made Aether question if he really made the right choice to challenge Signora. He was controlled by his emotion at that time, and again, when under strong emotion he is prone to make rush decisions.

Thank you, will be using this one!