Level One Espers
Belias, the Gigas (Tomb of Raithwall).
Mateus, the Corrupt (Stilshrine of Miriam).
Adrammelech, the Wroth (Zertinan Caverns, in Athroza Quicksands, closest from Ozmone Plain).
Zalera, the Death Seraph (Barheim Passage, obtain the Barheim Key in the Dalmasca Estersand).
Level Two Espers
Shemhazai, the Whisperer (Giruvegan).
Hashmal, Bringer of Order (Pharos at Ridorana).
Cúchulainn, the Impure (Garamsythe Waterway, complete "Lost in the Pudding" and use the key to reach Cúchulainn).
Zeromus, the Condemner (Stilshrine of Miriam, obtain the Stone of the Condemner from the Acolyte on Mt. Bur-Omisace after defeating Judge Magister Bergan).
Exodus, the Judge-Sal (Mosphoran Highwaste, after the Salika Gate is fixed, solve the floatweed puzzle).
Level Three Espers
Famfrit, the Darkening Cloud (Pharos at Ridorana, fought alongside Doctor Cid).
Chaos, Walker of the Wheel (Necrohol of Nabudis, must defeat Fury and Humbaba Mistant).
Ultima, the High Seraph (Giruvegan, Great Crystal).
Zodiark, Keeper of Precepts (Henne Mines, command at least ten other Espers, finish all Jahara based hunts and speak to Geomancer Yugelu in Jahara).