I can't say which Pokémon I always walk with, because everytime I change the leading Pokémon. :)
When I at Bill's House (Route 25), I talk to Ampharos, it has wires and scraps of paper in it's mouth.
Then at Route 33 (raining route), when I bring my Mamoswine, he looked sort of angry (cos it is a ground type, ground hates water!)
Then at Route 47, most of my Pokemon is scared of the cliff... But when I take a flying-type Pokemon like Lugia, it soars gracefully in the sky.