Sometimes, Sean resents Martin for being real. Sometimes, Sean wishes they could see him too.
Don't get him wrong, he loves Martin completely, loves him like a brother (father? Imaginary relationships were confusing). But that was all he had in the real world. As his imaginary friend, Sean had all of Martin's love for the Moones, but he would never really know them. He would never congratulate Fidelma and Dessie on their wedding, or hold little Rose, or keep girls from bullying Siobhan, or even keep Martin safe for that matter.
And he worries. He knows that Martin is an exception (Padraic too), growing older and holding on to his imaginary friend. He knows that Martin could, and maybe should, outgrow him any day now. Adults around Martin worry more and more when they think Sean is around, when they think he still exists. And he knows that one day, they wouldn't have to worry any more. He's seen it happen before. Sean's not sure what will happen when Martin is done with him. The others just... ceased to exist. Like they were never there (Were they?). He would vanish too, and Martin would go on. He'd be just fine without Sean.
But still, Sean knows Martin loves him; how could he not? They were practically one and the same. Martin always comes back for him, never lets Sean disappear for more than a few hours. Even though he was "too old" for an imaginary friend, Martin still draws cartoons of Sean, tells his stories. Even if he was hidden away, Sean's image, personality even, could live on after he was gone.
But that was the future. There was plenty of time to worry about the future later. Now? Now there were endless adventures to have and life to live. He and Martin, side by side.
And most of the time, that was more than enough.