This is one of the penalties of being a Pantzer, (a writer who writes by the seat of their pants). Someone can just pop out of the woodwork at any time and then they become a problem.
Is this going to be a spear carrier, or someone of importance? A Pantzer won’t know this so the character has to be developed outside the story. It is not as simple as what they look like and if they are good or bad, oh no. A whole profile needs to be worked up to give their entire history and therefore the possible reasons behind anything they might do. None of this will probably appear in the story, but it must be in the writer’s mind as the words unfold.
Another matter of major importance for the character is are they expendable? If they are not going to be useful further down the line, then the answer is yes, they are. In which case the character has to be adorable to wring out the maximum amount of angst when they die, unless, of course, they are a bad guy, in which case they have to be even more carefully constructed so they don’t appear like a cartoon bad guy. Even bad guys have needs, motivations and wishes, so there are many shades of gray between black and white.
The major trick is to determine if this is a good guy or a bad guy and sometime this is going to be ambivalent. Not that is part of the fun, the not knowing until it happens.