Not just making them be one-off character.
Don't just use them once and then throw them away. Give them more screentime in normal day-by-day interactions for instance. Your more likely to remember someone you've talked with five times than someone you've talked with twice after all.
Have the side-character have some sort of effect on the MC. For example the MC hasn't drunk any alcohol yet because the alcohol he tasted as a kid tasted weird. He encounters some random friendly drunkard on the day he went out with his friend to a restaurant. He strikes a conversation with the MC and once he hears that he hasn't drunk any alcohol yet despite being in university. He tries to have him drink some but MC rejects him and the drunkard backs off.
Now several weeks after that MC visits some random bar after some random heartbreaking incident because he was hungry. He encounters the same drunkard from weeks back. The drunkard recognizes MC as the kid that doesn't drink. MC protests that he isn't a kid but the drunkard says that anyone that hasn't drunk any alcohol is a kid to him and makes fun of him.
Drunkard feels that MC is depressed in some way and asks about it saying that it's good to get things off your chest before it's crushed by the weight. MC talks about the incident. After that drunkard offers MC some alcohol saying that it would be good to forget everything and just set loose. MC accepts it.
MC thinks it tastes weird and bitter. Drunkard laughs goodheartedly at MC and says that he's still a kid.
Since then, though he still doesn't drink much, whenever something sad happens MC drinks some alcohol.
Something like that. Have the MC be influenced in some way by the side characters. I'm not saying to change the MC to someone completely unrecognizable from him at the beginning of the story. Just have them influence him in some way.
It can be some habits, like spinning their pen when they focus after learning it from a side character.
It can be some hobbies, like gaming for instance.
They can develop a taste for this particular food after being introduced by a side-chara to it or anything else. Just have the MC be influenced by them in some way.
I did not think that it would get this long. Much less expected to write a whole story for this. I suprise myself sometimes.