Short answer is: whatever way works for you.
But let's get down to the nitpicky bits - why is your main character the main character? Why, out of everyone in your cast, did you choose him as the central character? What makes him interesting? You say he feels plastic - what is he lacking that the other characters aren't lacking?
You can sit here all day and answer questions about what his favourite food is, and what his favourite childhood memory is, but all you'll be left with is a list of facts which isn't always a useful thing. You want to know how your main character fits into the story, and most of all, you need to care about him. He needs to be more than just the story-function.
For example; my main character is my main character because the trigger-event for the entire story happened to him - but it also happened to another character in my cast. I could just as easily have made them the main character and told the story from their point of view, but I picked him because ultimately, the impact the trigger-event had on him was a lot more shattering, and he has a more interesting emotional journey to go through. He better represents the central themes of the story, and his character arc is more complex. He's also interesting to write, because he has to balance his responsibilities, his need to go on this (physical) journey, his ASTOUNDING grumpiness and wish to be left alone, and his underlying decency of character.
You said you've been going at your main character's personality, but you can't come up with anything - it sounds like you came up with the plot first, and the character second? I usually do it the other way around; I come up with the character, and then craft the story around them. If you came up with the plot first, and the plot is what matters, you might want to look at what kind of main character the plot needs to make it work, and then sort of work backwards from there.
Say you have a story that demands your character be an adventuring hero - then you can sit down and figure out why your character decided to go on the adventure; why did they run off to fight dragons instead of settling down to be a baker in a quiet village, or whatever choice it is they've made. Is it because they're restless and impatient? Is it because they never felt at home in the village? Is it, oh cliché of clichés, because a dragon killed their mother? Are they the sort of person who goes adventuring for fun, or are they more responsible and serious? And so on and so forth.
Also, there is absolutely nothing stopping you from keeping this character the way he is, and merely shifting the focus to someone close to him instead. That way, he still gets to exist within the story and do what the plot demands he do, but we the readers get to see him do it through the viewpoint of another character. Like, he's off fighting dragons, and we exprience that through his best friend who has tagged along and is worried about him/cheering him on/angry at him for going on an adventure/whatever. There is ALWAYS more than one way to tell a story, so if this guy doesn't feel up to scratch as a main character, you can shift focus without having to reorganise your cast.