E-104 Epsilon, more commonly known as GUEST, is an orange robot in the E-100 series created by Dr. Eggman. He only appeared in Sonic Adventure.
Eggman sent Epsilon along with his brothers, E-103 Delta, E-105 Zeta, and E-102 Gamma to find Froggy, a frog with Chaos's tail. Like Delta and Zeta, Epsilon brought the wrong frog to Eggman, who sent the three away.
Later, Gamma embarked on a mission to destroy the E-100 series to release the animals imprisoned inside them. He tracked down Epsilon at a volcano and destroyed him in battle.