He is able to be extremely secretive under his friendly, charming persona, and that mask enable him to hide what he thinks other people might not like to see.
It takes a lot of work to be considered a close friend of his, ie, to be trusted enough by him for him to actually show more of himself than he does to the world.
He is in Nicolai's art class, and they are next door neighbours in the same apartment building, so he is very close to the other Guardian, who is, perhaps, even more secretive than he is.
Alexes gets jealous very easily, but he can usually control it enough unless something in particular provokes him. In a way, his hot temper helps regulate his privacy, because he doesnt need to hide that aspect of himself, nor does he have to restrain it when other people see it. They expect to see it in him if they push him far enough.
He is strangely and unexplainably attracted to the twins, but since he and Arical dont get along too well (mainly because Arical's got his sights set on Nicolai, who is also Alexes' boyfriend), Alexes has started to like Devika instead.
This pushes him and Arical further apart until they get huge amounts of enjoyment in teasing each other. Other people say that their teasing is malicious, but its a peculiar form of friendship, and it seems that Alexes quite likes Arical at the end.
Spoilers (highlight to read)
Alexes doesnt remember too much about his past, and for good reason. He was also a citizen of Caer Maellyn, except he didnt have anything remarkable in him that gave him leeway in court like Arical. He was a simple soldier, good with the bow and arrow, but excellent with the sword. He was sent away to battle one day with the rest of the King's troops, and he never came back. They assumed him dead, and gave him a proper ceremony for it.
He was not dead. He was being tortured in Raegwyn, the main foe of the King. In the dungeons of the main Castle Addrycyn, the largest castle in the enemy country, solely for prisoners of war, he was being maimed and persecuted for loyalty to his King. Eventually they killed him.
Following rules for such deaths, a spirit took rewarded his loyalty with a new life. Unfortunately, it was not another immortal life, because that was against the rules. Instead, he was allowed to join his mortal, who was, by that stage, 20 human years. In joining, he gave up his memories willingly to be able to forget what had killed him, and in doing so, his mortal body lost some memories at the same time.
He does not know he was also an immortal, and he does not take kindly to the fact when he eventually finds out.
End Spoilers
Alexes is very loyal to those he trusts, and would do nearly anything he could to protect them. The only reason he continues to allow himself to fall for Devika is because he can plainly see that Nicolai is falling equally hard for Devika's twin, and that'll stop either of thm from hurting each other.
Of course, with such a hot temper and an almost regretably quick mouth, he will continue to be himself.
At least, so much as he will allow the others to see.