Sometimes NPCs from a characters background might take the place of a side-character or a important, prepared NPC.
Try to see fits, and Ask players if they’re okay with it.
Reference
Five Simple House Rules for Character Creation
Monty from the Dungeon Dudes
As a dungeon master you’ll create dozens of NPCs. A great habit to get into as a DM is to mark key NPCs with an asterisk, to mark them that these are great NPCs to have their identity replaced by identities proposed by the player.
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