Ingredients for a character on the side

Your main character needs people to interact with.
Some are minor, but others are more important

Ingredients for a side character

The reader does not get to see their viewpoint
Introduce them twice - the first time in passing and the second time more clearly so the reader thinks - oh yes I've heard his name before.
The side character doesn't know they are a side character, they are the hero of their own story and they need an arc through the story.
They are important to the story.
Never in a scene alone.
Around enough times to be more than a minor character.
They have their own voice.
They have flaws.
Generally have one clear trait- an allergy, fashion accessory etc
They may be linked to a particular place.
They add more conflict to the story by annoying the main character, pushing their emotional buttons.
They can be someone for the main character to chat to.
They are a support, sidekick etc.
Not too memorable.
They can be killed.
They can become a main character in a sequel.

Enjoy and have fun creating.