Like I said, I dropped it after Kaban appeared, so I really dunno what happens afterward but I assume it's more of the same (the discussions I've seen seem to confirm that)
She literally does nothing. There was a microscopic tease that Serval was the same Serval as Season 1 who lost her memories, but literally nothing happens with it when Kaban shows up, her blandness almost seems like she's just given up on the world. It's just a complete insult to everything before it. Kaban also seems to have no attachment to Serval at all. Why the hell did they even connect it to season 1 at all?
It always kinda tried to clumsily hint at something, but it anticlimaxes it every single time. The episode before the Kaban episode ends with a dramatic reveal that it's a grown up Kaban, and she saves the original party from a wild Friend (called Beast, which I assume becomes the final antagonist since it is more or less following the party around since the start) by throwing a paper airplane, and it's all pretty cool seeming.
But then the next episode, all she does is sit in a lab with the two owls (who also lost all their fun character bits) and spouts some exposition for the plot that anyone could have already guessed. Then the "conflict" of the episode doesn't even really concern her at all, and the end is just her waving with dead eyes like she's probably a few bottles of whisky away from just staying in bed 'till she dies. None of the cool inventive Kaban that was in S1, or even the image of "the charismatic human that can control or befriend animals" that seemed to be an overarching theme for the second season. She was just some girl in a labcoat.
If it were in a vacuum the show would have been brushed aside as a kinda airheaded kids show about animal girls but the fact that it makes so many callbacks to season 1, yet faceplants every single time it does, makes it seem almost mean spirited. Not to mention that the animation quality is almost worse than season 1's irodori brand animation, and the character models look much more awkward despite being more smoothed and clean.
to summarize, it's the soul-soulless meme but pretty much true