Ha well he was actually at Athletico Madrid rather than Bilbao. Although as a Spurs fan, I very much sing his praises.
One of the greatest things about Athletic Bilbao is that they only have players from the Basque region (north east Spain and south west France). They're also the third most successful Spanish club of all time with 8 league titles and 24 Domestic cups and never relegated. To have done this whilst also having all their squads essentially come from their version of Wallsend Boys club, really makes them a special club. Unlike Barca, Athletic truly are 'more than a club'.
Amongst the Athletic fans I know, there's a real affinity with the north east of England, given historical ties (I wont mention why they wear red and white). Also the club was founded in the 1800s by mineworkers from NE England.