I am in a tough situation and could use advice.
I co-GM a campaign. I am subordinate to the "main" GM, but I wind up doing more work on storyline planning, quest planning, encounter-building, acting out NPCs, and running scenes, but the main GM contributes as well and has the final say in all things.
The group consists of the main GM, the co-GM (me), and three players. We have been getting along fairly well; the group has had smooth chemistry. I try to cater to each PC, make their backstories and NPC contacts relevants, put them in situations that are relevant to their goals and beliefs, and so on.
However, a few days ago, the main GM brought in a friend of theirs as a new player for the campaign. The players were fine with this, but I contested this, because my GMing style works best for smaller groups. The main GM insisted on having their friend as a player, so I reluctantly accepted the new player.
I heard out what the new player had in mind for their character. The main GM liked it. Conversely, I disliked the character's concept, goals, personality, and even appearance. (I am not going to go into specifics so as to avoid bias.) For the past few days, I have been trying to negotiate with the new player and the main GM; we have reached a few compromises, but the new player has been adding new details that make me dislike their character even more.
I have made my distaste for the character clear to the group. Everyone, the main GM and the new player included, has more or less shrugged their shoulders and hopes that things will work out.
How should I handle this as I plan out sessions, act out NPCs, and run scenes?
I try to be fair to the PCs and distribute the spotlight equally, but I doubt that I will get used to the character; if anything, my resentment may grow over time. How can I fairly GM for a character I disapprove of on many counts? If I ignore the character and leave them to the main GM to accommodate, that will come across as forced and awkward.