As stated above, its not a single guy, that would be mad.
And well Andrew Gower and his Co-Founder brother Paul Gower aren't really making "all that money" anymore from Runescape, considering they both already cashed in majority of their shares of the company, its basically all investors now making the shots, hence all the money-grabbing updates like Squeel of Fortune.
As i quote from Wikipedia:
"In December 2010 he left the Jagex board of directors. He no longer has holdings in the company. As of November 2011, he is not listed under the credits for RuneScape. Since leaving Jagex, Gower has founded a new gaming development and consulting company, Fen Research, of which he holds 90% of the shares. "