hmmm, a pixie ghost?
excellent idea, felix. this character concept was inspired by the palladium fantasy monk class, arguably the most irritating class in the entire palladium (including non-fantasy) system. rifts, tmnt, heroes unlimited, superspies, robotech... this class tops the irritation list. worse than a kender in some views.
the palladium monk is bound by class features to act out the rp vows of non-violence and aggressive pacifism. as in you have to try to stop everyone from fighting and try diplomacy instead. perhaps foreseeing the likely pc/npc response the designers gave the monk a massive ac, resistances or immunities to almost everything and anything that could kill one. a monk is typified as standing between two would-be combatants preaching the perks of a non-violent lifestyle, and would they please consider diplomacy to fighting. while madly dodging. and making the others miss each other.
add in some various anime/manga/other literature sources of irritating old men and their foibles, plus the fun of seeing my pcs (or my dm, when it comes that time) try to squash this character. (assuming dm fiat doesn't finally kill 'im off, that is.)
the irritation is going to rely mostly on rp situations - blowing their cover, leaking information, causing them trouble, and managing to possess items or information necessary to various side quests (i wouldn't make it the main plot... that's just too mean.) think loud, obnoxious, poor personal hygiene, halitosis, no sense of personal space or boundaries, always rummaging through their stuff... some cross of a persistent begger, yoda before luke knew who he was, various old-men-masters in cheesy kung-fu flicks, and a few anime/manga influences minus the apparently obligatory perv-ness.
i imagine this will result in rapidly escalating attempts to capture/kill the offending character/npc. ^^ so he needs high survivability.