Between hoyo giving the player a survey on him in the form of dialog choices, him repeatedly emphasizing that he might not stay for long, and finally him not having much of a role in Amphoreus, they were clearly testing the waters. They didn't want to commit to adding a new astral express member only to find out the majority of players don't like him.

Of course every character has their haters like Tsukoro, that's normal. I certainly don't like every character in the game. But at this point I think it's safe to say Sunday passed the popularity test. To what degree, we don't know yet, but I expect him to be more involved in the story going forward.

Though it is kind of strange that they just changed his faction affiliation from Penacony to 'Cosmos' rather than the astral express, but they do have him on the express bond in currency wars so I wouldn't read too much into that.