If you look at the wisdom scores - Jan actually has a pretty good score at 14. Among recruitable NPCs in BG2, only Aerie, Sir Anomen, Cernd, Dorn, Keldorn, and Viconia are higher - all from divine spellcasting classes. Stat-wise, the strongest high int/low wis combinations are Edwin (18/10) and Nalia (17/9). Edwin has his megalomania; Nalia's low wisdom mostly comes across as naiveté.
And when you look closer, there are a few moments when Jan gets serious. He's definitely putting on an act to mess with people.
The tinker gnomes of the Dragonlance setting for AD&D helped to popularise the gnomes as oddballs trope. Their cultural traits included immensely long names of entlike detail, lifelong devotion to particular projects, and fantastic inventions with crippling downsides.
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And when you look closer, there are a few moments when Jan gets serious. He's definitely putting on an act to mess with people.
You have been warned.