I am starting to play a certainly chaotic, not necessarily evil (depending on your viewpoint) character, which is looking for a reason to go adventuring with a group of magic-using, but otherwise rather mundane characters (i.e., regular village folks going out in the world to solve some plain-folks problem).
My character is from a quirky system and basically comparable to an Ilithid in D&D lore; I'm also nicking parts of the D&D background for him (for his city, deities etc.) although the rest of the world is not D&D based. So, basically a mostly humanoid, tentacled, psionic brain eater from a (from their viewpoint) very advanced high culture based on arts, literature, architecture etc., but not popular with other races due to their brains being on the menu once a day. Unlike the way Ilithids or Mind Flayers are usually portrayed in D&D, he looks pretty normal, except for the tentacles and slightly more reptilian eyes and nose (think Voldemort). The rest of his physique is bog-standard human.
My toon has pretty nefarious religious long-term goals (basically, to make the world ready for the return of one of the Ilithid deities through rituals, ancient magic tomes or whatever) which makes it necessary to go out and about and visit places, possibly along with a party of regular folks. Aside from that, and aside from their cities regularly employing huge numbers of thralled sentients as food, and aside from having to consume a brain more or less daily, there is nothing particularly evil about him (if you are from the Ilithid culture, obviously!) on a personal, small scale. He is intelligent and charismatic, and as part of a group certainly able to play the good samaritan if it benefits his own goals.
Day-to-day, his only evil aspect is the brain-eating, and he is perfectly capable of not feasting on the small group he's gallivanting with; and to hide is eating habits as long as possible; and not to eat important NPCs. Otherwise, he is not cruel, he does not look down on his food, he only kills what he eats, etc.
We had our first session, and it worked out great; we accidentally met in a common plight and quested our way out of it, and the world now is ours to explore, with each of us having their little or large individual goals. The way the DM runs his campaign is that all of this is not known to the other players, and there is plenty of 1:1 going on between sessions. We players know each other for a long time, are all mature, and there is little risk of anything going wrong in out-time, but everybody only knows about the other characters whatever we learn during play.
As mentioned in the answers to a vaguely similar question, or this one, I am fully aware of the possibility that the other characters will reject this one when they learn his true nature, and am prepared to "hand him over to the GM" and/or maybe even being fought/killed. Those two questions focus on evil characters, but as laid out above, I do not consider mine to be evil per se, although other inhabitants in the world would certainly do so.
Have you played something similar in the past, and do you have some hints or inspiration how to keep this running in an interesting manner (for everyone involved) for as long as possible without breaking disbelief constantly or at all?
To try and focus the question more: how to play a character which on the one hand is not directly adverse to the other characters (he has no nefarious intent towards them, has no will or reason to harm them, and could for all intents and purposes be a valuable long-term member of a roving band of adventurers), but if his true biological nature would be discovered would probably lead to immediate and total disgust and rejection if the other characters are played true to their nature? Hiding it forever does not seem to be an option for practical reasons (i.e., the question "why do you never eat with us" would come up very soon; his tentacles and brain-eatery is also a valid combat technique, and the first time he used it without hiding it very well would also be quite the event...).