From Sonic Retro
E-121 Phi refers to any one of several clones of Emerl created by Dr. Eggman. The Phis' ability to copy attacks is identical to that of Emerl, but since the ability is only powered by Chaos Emerald shards, as a whole it is weaker than Emerl's.
The original basis of Phi's creation was on Dr. Eggman's misconception that Emerl was broken. However, after Eggman realized his mistake, he sent the Phis to capture Emerl. Emerl eventually destroyed all of the Phis, and collected the emerald shard that the Phis used to power themselves.
The Greek letter (Φ) is not used to refer to Phi, unlike the other E-100 Series robots. This may be because the Greek letters are never used to refer to them in the standard game text in any of the games.
It should be noted that Phi is the 21st letter of the 24-letter Greek alphabet, however the naming sequence starts with 0 and ends with 23. Thus, if following the sequence of the others, E-121 should be the letter Chi, with E-120 being Phi.
|Dr. Eggman's E-100 Series of Robots|
|E-100 Alpha | E-101 Beta (Mk II) | E-102 Gamma | E-103 Delta | E-104 Epsilon | E-105 Zeta | E-121 Phi | E-123 Omega | Egg Keeper|