Morn is one of the best and funniest characters ever. I love his obvious fake suit, his inability to change his facial expression from that rather "who me?" and resigned/depressed look.
He's always seen in the background doing odd things, showing up at odd times, such as behind Jadzia's couch or boarding a runabout to evacuate DS9 (during the Circle trilogy).
And other characters act as though he's a multi-talented, excessively talkative, and highly volatile (he supposedly ran through DS9 naked screaming "we're all doomed" and inadvertantly injured Quark in his hysteria when Quark subtly mentioned the looming Dominion/Cardassian threat.) Yet his shown personality appears *so* resigned and gloomy, it's impossible to imagine him engaging in any of his attributed antics (plus he never once says anything on screen or exhibits any emotional expression — Mark in his bulky Morn suit could just barely rotate!)
Another funny thing is there's no reason or given explanation why he'd spend *all* of his time aboard DS9, and more specifically, Quarks. In "Who Mourns for Morn?" it turned out he was previously participated in a big-time heist, which I suppose explains why he's independently wealthy enough to sit around all day at Quark's. He appears to have nothing better to do. Ever.
He was one of DS9's brilliant displays of committment to continuity and restraint. I'm *glad* they never caved and showed him doing anything useful or finally speaking on screen.