Siegfried Erkle can be found on the 2nd floor[UK] of the Legends' Guild, which is accessible by going up the western ladder on the first floor. He runs the Legends' Guild Shop of Useful Items, which is the only source of the cape of legends, shield right half, and purchasable mithril seeds, maze keys and dusty keys.
He is the brother of Radimus Erkle, the Grand Vizier of the Legends' Guild.