E-101 Beta (ベータ Bēta?) (β) was a silver and black bipedal robot that was created as a prototype machine for the E-100 army by Doctor Eggman. Its original design featured a Jet Booster, and twin Guided Missile Launchers on its arms.
Dr. Eggman defined Beta as E-102 Gamma's "big brother", and decided to pit Beta and Gamma against each other to determine which robot would be fit to serve under him on the Egg Carrier. Gamma ultimately went on to defeat Beta in combat, earning his right to serve aboard the ship, and his design was replicated for the remaining E-100 Models - Delta, Epsilon and Zeta.
After convincing the Doctor to let it stay aboard the Egg Carrier as well, the obsolete Beta was relegated to the storage area to be used as "spare parts". Unhappy with its flawed design, Dr. Eggman began work on completely remodeling it into E-101 mk2 (E-101 改). The parts left over from Beta's remodeling, including its Jet Booster and Missile Launchers, were apparently used on E-103 Delta.
During its time in storage, Beta underwent extreme reconstruction into a highly advanced "Mark 2" (mk2) model, taking the form of an airborne fighting machine enhanced in almost every way over its previous model. Besides being much larger and stronger than a regular E-Series robot, some of its new features included the ability to launch more advanced Guided Missiles, twin Laser Cannons in its arms, and large shield-like spiked gauntlets used for both defense and high-speed charging attacks. E-101 MKII navigated with jets and a tail-like rudder that granted it high-speed flight and levitation, and employed an advanced tracking system in its head, taking the form of a single green camera "eye".
E-101 mk2's improved fighting capabilities granted it the ability to create temporary rainbow-colored force fields, fire spreads of several (ranging from 2 to 10) Guided Missiles to chase down moving targets, and fire powerful energy blasts from great distances using its new cannons, which would cause massive explosions on contact with the ground.
It was also much more agile than its previous form, being able to dodge missiles at extreme speed (leaving afterimages in its wake), and even still, could also simply knock them away by backhanding them with its shield-like gauntlets.Gamma and the improved Beta faced off once again aboard the downed Egg Carrier, as part of the former's crusade to "rescue" his brothers from Eggman's control. Beta, despite its numerous improvements, had its weaknesses exploited, and was defeated by several well-placed missile shots. After its defeat, the critically damaged E-101 mk2 fell to the floor of the Egg Carrier, lying motionless on the ground.
However, with the last of its power, Beta suddenly blasted Gamma with both of its cannons at point-blank range as he approached, causing fatal damage to its younger brother and exerting its power supply at last, finally exploding in a dome-shaped fireball as Gamma limped away. Through this final act, it ironically assisted in completing E-102's mission by freeing the birds trapped within both of them.
In Sonic X, E-101 mk2 is referred in Japan as "E-101 Kai".
E-101 Beta appears in Sonic X. In the Sonic Adventure Adaption of the show, Beta mirrored several key points in its original story line and also gained more scenes by taking most of E-105 Zeta's role from the game.
One of these key features that was not present in Sonic Adventure is Beta's ability to speak. Like ZERO in Sonic Adventure, E-101 was also unable to, or was at least not shown to speak, and could only respond by "Body Language"- in its case, interrupting Eggman and giving signals via movement and its blank facial expression. Beta also goes out to find Froggy along with the other E-Series robots, something which originally Zeta had done- in the game, Beta was already undergoing its modification process to turn it into E-101 MKII.
Like the other E-Series in Sonic X, Beta's new voice was very high-pitched and according to fans, unenthusiatic. Much criticism was given to the voices of the E-Series robots, as the original voices from Sonic Adventure were relatively deep to a low medium range. Also in Sonic Adventure, the robots spoke in a manner that while still being slightly robotic, was very intelligent and much more human than drone-like.
When E-101 Beta is destroyed, a gray Flicky with red eyes emerges from the robotic armor sealing him inside. After Gamma's story, he flies off with Gamma's Flicky. He also appeared in Amy Rose's story in Birdie's pendant showing Gamma and Beta as Flickies.
- Beta and Gamma are mentioned in Sonic Heroes; when Team Dark gets an E rank, Omega would occasionally say "I couldn't even beat Gamma or Beta."
- It is assumed that Beta's discarded Missile Launcher arms were used on Delta.
- Beta is one of two E-Series robots in Sonic Adventure to receive a physical upgrade (the other being Zeta). The remaining E-Series, including Gamma, only received enhanced Arm Cannons.
- Strangely, in the ending shots found in the credits of Gamma's story mode, any time Beta is seen he does not feature his jet booster or twin arm cannons, appearing as a black version of E-102 disregarding his specific numbering and lack of targetting sight.
- When found in the middle of his reconstruction, Gamma looks down and sees Beta's discarded parts on the ground, oddly including Gamma's missile launcher despite Beta having twin arm cannons.
- E101 MK II is fought on the same place as Chaos 6 and ZERO.