E-104 Epsilon (Ｅ－１０４ "イプシロン"?), or E104ε, is an orange robot modeled after the E-101 Beta model machine. E-104 Epsilon is equipped with twin STINGER missile launchers on the arms, which are useful for taking out moving air targets, and a Jet Booster.
Like E-103 Delta, Epsilon was first a recolored clone of E-102 Gamma, but after being discarded, he was equipped with upgraded twin stinger missile launchers. After Gamma was proved superior, Epsilon was teleported to an unknown location along with Delta and E-105 Zeta. Later, Gamma found Epsilon in a lava-filled cave in Red Mountain on Angel Island. The two fought, and Gamma emerged victorious. A bird inside was released after Epsilon was relieved of duty.
- Epsilon's Sonic X design is near identical with his game counterpart, with minor alterations to his double cannons.
- Epsilon is the fifth letter of the Greek alphabet and E-104 Epsilon is the 5th E-100 robot made.
- Epsilon also seems similar to the robot, Isaac from the Archie comics in terms of color. Though Isaac also looks like Gamma and colored a glossy gold instead of Epsilon's rusty orange color.
- His Jet Booster seems to let him fly for a short time, instead hovering of like Gamma's.