I've been wanting to respond to this for awhile, but haven't got a chance to fully get to it yet.
So, in
every story, other characters
need stories too. They need backstories, conflict and challenges, as well as goals. Just think of side characters as the people around you in your life. They have stories too. They don't just exist to move your story forward. They do move your story forward, but they have lives outside of it, and sometimes they can't show up, or might even disappear forever.
On the topic of
Thing's Story, I want to give an example of one of my stories,
Kiara's Adventure.
The story is named after her. She's the protagonist, obviously. And she's on some crazy adventure to find this person who could very well be dead. But, she's not doing it alone.
She can't. She has others surrounding her, some she's dragged along, helping her - as she intended, but also hindering her in places she doesn't want. It's not all about her, even if that is what she believes in the beginning of the story. They have feelings, opinions, goals, desires, and so much more.
Coming from a story named after the protagonist, Kiara, in
Kiara's Adventure, here are some of the side characters:
- Vladimir: Vladimir is Kiara's best friend. He has the knowledge and skill that will help her achieve her goals and mission, plus he's the only 'adult' who could help her pass through things without being suspicious for being a minor. But- he's a person, not a robot that'll do everything she says. He doesn't think they should go on this adventure, but instead, he goes because he's afraid that something might happen to Kiara. He has his own backstory, having lost his father, witnessed his mother be nearly killed, and having been held hostage for nearly a year. This then affects him in several ways throughout the story and eventually becomes something he must overcome, or else it'll destroy him.
- Kenoi: Kenoi is Kiara's older brother. He's supposed to be watching Kiara, but also, he's the one person who will tell their parents if anything happens. He's dragged along to ensure he doesn't tell. He's quiet, and doesn't really oppose, which gives him that 'robot' feel, but that doesn't mean he is one. He's always the first to oppose Kiara, he hates her means of travel, and eventually, he starts listening to the wrong person and runs away, putting them all at risk.
- Jintao: Jintao is the mysterious one-armed man that Kiara meets along the way. He quietly shares his disapproval of Kiara and her company, but seemingly goes along with everything Kiara wants. Only, little does she know his goal has always been to sabotage her plan. Why? Because he's afraid. Kiara's goal is a threat to him and possibly to everyone he cares about. He's not the antagonist, but with Kiara digging for the very thing he thought he had disposed of, which is now a risk to his life, he'll deceive his own friends into doing something rash.
- The Safari Family: The Safaris are a bit of background characters, but that doesn't mean they can come along without a story either. Their parents were both brutally murdered, with the youngest, Jack Rabbit, having witnessed it first hand. They're at constant risk that the murderers will return, even though they did take in Kiara and her company. Eventually, the murderers do come back, and they must face them (alongside Kiara), or else die by the explosives they're held hostage by.
- Kearns: Kearns is the antagonist of the story. It's very important that they have their own stories too, and Kearns does. Like in most cliches, Kearns has a scar on his face, or more specifically, he is missing an eye. But this isn't about being a cliche bad-guy. (And yes, I'm not a fan of cliches and didn't mean for him to be a cliche.) He lost his eye trying to save the life of his last captain. He failed miserably, but he's not done fighting. He intends on rebuilding what his captain had lost, and blowing up his enemies is the first thing that will ensure he's unhindered. Kiara gets in his way because he knows something about the man she's searching for, and he might even know where he is, if not be trying to blow him up too... or working alongside him.
These are all characters in a story named after the protagonist. They are moving Kiara's story along, though some are hindering her, yet each of them are their own person. Some have their backstories getting the best of them, and others it's their future goals driving them. This is all important when it comes to writing side characters. Unless they have their own stories, they're nothing but robots serving your protagonist and hurting the story instead of adding to it.
So, back to Stephen. He can and he
should have his own backstory. Let it play out. Let others see he's a real person and not just a robot serving Thing. And same with any other character that you put into your story. You might not need to develop their stories right away and it might be more of something for a future draft, but make sure they're not just robots serving (or even hindering) Thing.
(Oh, quick thing to note about my above list: All these characters were developed from my freestyles. Not all characters come with stories so easily. It's just something that fit my story and it's a good example that the side characters need stories too. Some of your side characters might not need a story so in-depth, and these characters in my list might not even show a quarter of their overall story.
Also, please don't copy any of my examples. These are all characters that it has taken me years to develop, and some will hopefully be in future (published) writing and will be copyrighted. I just wanted to share all of this to hopefully help you and anyone else who needs some help with their side characters.)
~Lacy Duckwing
He was so boring 
But once I came up with it, no joke, EVERYTHING suddenly made sense. Everything I'd already written, I can read through and suddenly see a motive that wasn't there before but I somehow still kinda put there. It really does make a difference. As I'm adding in characters, I don't intend to give them too deep backstories, the only other one who needs that is Theo, who may or may never show up 
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