If you pick a base character then the prime gets removed as well, the better version cant exist without the base version, however if you remove a prime version then the base character can still exist. My personal pick would be Valkyr due to how [IMO] useless she seems
umbra would still be considered as a prime version, because he came later in the game and was based off of excalibur himself, so he would get removed if base excalibur got removed