Hi Black Cat!!
Do you have a subplot in your story? I added one or two subplots somewhere around revision #12 and it was a fantastic way to beef up minor characters. If you don't have a subplot, you could add one of those and your doomed tertiary character could figure prominently within it.
You could also smash two characters together. My critique group always asks me to do this, get rid of characters, smash them together, etc, so they won't have to remember so many. If you have a lot of characters, maybe you could smash your tertiary character with another minor figure.