E-121 Phi

E-121 Phi (ファイ), or E-121 “φ”, is the collective name for several robots built by Doctor Eggman based on the ancient Gizoid, Emerl. Dr. Eggman took the Gizoid's data and copied it to create the Phis. They are massed produced, though only 14 are seen. The Phi robots are powered by Chaos Emerald Shards, rather than whole Chaos Emeralds.

Abilities and Differences from Emerl
The only physical difference between the Phi robots and Emerl is Phi's grey hue and brown eyes, as opposed to Emerl's yellow and blue. The Phis' abilities are stated to be similar to Emerl, however the Phis only use a complete moveset of one of the playable characters, making the Phis robotic, grey-scale versions of the playable characters. The majority of the Phi robots appear to be of the same intelligence level as Emerl when he had only absorbed two Emeralds, however the last two Phi models you encounter, #13 and #14, were shown to have much higher intelligence.

#1
The first Phi model created by Dr. Eggman, it is fought in Rouge's episode at Battle Highway in Central City. Eggman attempts to use this Phi to defeat Rouge, however Rouge effortlessly defeats the Phi and claims the robot's Emerald Shard as her own. Unlike later models of the Phi, #1 uses basic attacks used by Emerl after first establishing a Link with Sonic.

#2
Like the first model of Phi, this one uses basic attacks used by Emerl at the start of the game. This Phi is first fought by Sonic outside of Tails' Lab in Emerald Town and is later fought by Emerl after he receives training. This is the first Phi fought in the game, as well as the first enemy Emerl fights on his own. After it's defeated by Emerl, it runs away, presumably to Gimme Shelter.

Phi #2 is later fought again in Night Babylon at Club "Rouge" alongside #3 in Knuckles' episode, unlike the first encounter with the robot, it now uses Rouge's move set. #2 is then completely destroyed by Knuckles and Emerl, leaving an Emerald shard behind.

#3
Phi #3 is fought in Knuckles' episode in Night Babylon at Club "Rouge" alongside #2. The two Phis are fought by Knuckles and Emerl, #3 uses Knuckles' move set. Once #3 is defeated, it leaves behind an Emerald shard.

#4
Phi #4 is fought in Knuckles' episode at Emerald Beach. After Knuckles trains Emerl with a one K.O. battle, Dr. Eggman arrived to take Emerl back by force. The two then quickly dispose of the mock Gizoid, obtaining another Emerald shard in the process. Phi #4 uses Tails' move set.

#5
Phi #5 is the last Phi robot to be encountered in Knuckles' episode. The player encounters the robot after pursuing Eggman to the Crossing in Central City. The Phi uses Shadow's move set and is fought like the previous three Phi models by Knuckles and Emerl. As with all Phi model robots, Phi #5 releases an Emerald shard upon defeat, which supplied the two with enough shards to recreate a Chaos Emerald.

#6
Phi #6 is fought in Amy's episode on Emerald Beach. After Emerl wondered off, he discovered the robot and attempted to fight it, however Amy arrived and scolded Emerl for running off and being rude to the robot, Amy then attacked the Phi for attempting to attack Emerl. Amy and Emerl then fought #6 alongside #7. Like Phi #3, #6 uses Knuckles' move set. After Phi #6 is defeated, Amy and Emerl obtain another Emerald shard, which prompts Amy to train Emerl in order to defeat more Phis and obtain more shards so that Sonic will respect her.

#7
Phi #7 is fought in Amy's episode alongside #6 on Emerald Beach. The Phi model is fought by Amy and Emerl and uses Chaos Gamma's move set. After Amy and Emerl defeated Phi #7, it leaves behind another Emerald shard.

#8
Phi #8 is fought in Amy's episode at the Signboard in Night Babylon. It is fought alongside Phi #9 against Amy and Emerl. Like Phi #5, it uses Shadow's move set. After it was defeated, Phi #8 left an Emerald shard which, along with #9's shard brought Amy and Emerl's shard count up to four.

#9
Phi #9 is fought alongside #8 in Amy's episode. It is fought in Night Babylon and uses Amy's move set in battle. Once defeated, it releases another Emerald shard for Emerl.

#10
Phi #10 does not appear in any of the game's episodes and the model numbers skip straight to #11 at the start of Cream's episode. Although the fate of #10 is never revealed, at some point between Amy's and Knuckles' episodes, Rouge obtains another Emerald shard, suggesting that Rouge may have the defeated the tenth model.

#11
Phi #11 is fought in Cream's episode ay the Laboratory in Gimme Shelter. #11 is fought alongside #12 against Emerl and Cream (although Cream does not participate in the fight). #11 uses Sonic's move set and, as with all of the Phi models, released an Emerald shard upon defeat, Emerl then gave the shard to Cream to hold as she found the shards pretty.

#12
Phi #12 is fought alongside #11 in Cream's episode in Gimme Shelter. Like #4, #12 uses Tails' move set in battle. Once defeated, the robot released an Emerald shard which was given to Cream.

#13
Phi #13 is fought in Cream's episode at the workshop in Gimme Shelter. Phi #13 is fought alongside #14 and is fought by Emerl and Cream however, as with the previous battle, Cream does not fight back and isn't present at all during the second fight. As with #2, #13 uses Rouge's move set, and once defeated, it released another Emerald shard. Unlike previous Phi models, this model, along with #14, is capable of fluent speech and cognitive thought, as opposed to previous models, which used speech similar to Emerl when had only absorbed two Chaos Emeralds.

#14
Phi #14 is Eggman's last model in the Phi series, and therefore, is the last Phi robot encountered during the story mode. It is fought in Cream's episode along with #13 and fought against Emerl and Cream, although Cream does not fight and is not present during the second fight. As with #2 and #13, this model uses Rouge's move set and it was defeated, it left behind the final Emerald shard in possession of the Phi robots.

Trivia

 * Phi is the twenty-first letter of the Greek alphabet. However, since the first E-100 Series robot was designated E-100 (as in the Greek Alphabet, Alpha is counted as 0, then Beta is counted as 1 and so on), the Phi robots should be designated E-120, and E-121 should be Chi. This error was acknowledged by Ian Flynn in the Sonic Universe blog, where Phi and Chi are listed by what their model numbers should be with the error noted in Gamma's logs.
 * Phi has a lower serial number than E-123 Omega, meaning that it was created earlier. However, Sonic Battle takes place after Omega's first appearance (in Sonic Heroes), as Rouge comments on Gamma and Omega looking similar. This might mean that Eggman was working on Phi before Omega, but he finished building Omega first. Eggman may also have saved the Phi model robots for a later purpose, such as collecting Emerl.
 * Other than E-102 Gamma's copies in Sonic Adventure 2, Phi is the only E-Series robot to have multiple versions of itself.
 * Rouge's move set is the most frequently used move set by the Phi models, as it is used by three separate models during story-specific events.
 * Phis are never encountered in Holy Summit.