Mr. Heckles

Mr. Heckles is Monica and Rachel's downstairs neighbour, an elderly man who constantly complains about the noise. He first appears in "The One with the Blackout", claiming to own a lost cat that Rachel has found. He appears again in "The One with Two Parts, Part 1" and "The One Where the Monkey Gets Away" before dying in "The One Where Mr. Heckles Dies". As a last spiteful act, he leaves all of his junk to "the noisy girls upstairs". He makes a final cameo appearance in "The One with the Flashback", set in 1993, where inadvertently causes Joey to be Chandler's roommate.

Example of Mr.Heckles weird behaviour:
PHOEBE: [opens apartment door and sees it's Mr.Heckles] No! Mr. Heckles, no one is making any noise up here!
MR. HECKLES: You're disturbing my Oboe practice!
PHOEBE: You don't play the Oboe!
MR. HECKLES: I could play the Oboe.

or

MR. HECKLES: quiet, your disturbing my birds!
RACHEL: you dont have birds
MR. HECKLES: I could have birds...

Let's say he's a strange man that, like Gunther, gives more laughs to the show.