"Neptune is very kind or possibly super mean and hates us. But that's okay. She's really funny. Neptune looks in no way like she's supposed to be here. But that's why she's here, of course."

From the wiki: Neptune is bossy and fashionable, and doesn't care what others might think of her or how her own actions might affect others. Neptune typically behaves as she would like to, with little regard for others' approval. While she can be nice to those she cares about, her tone comes off as rather abrasive and sarcastic most of the time.

Neptune should sound deadpan and uninterested in most lines, but it is important that the voice actor is still able to bring emotion when needed.