The speed remains the same thanks to Ivy and Cervantes [Superhuman with Supersonic+ combat speed and reactions], everyone kind of keeps up with the most of the others char. sans for Algol and Night Terror [Who are Subsonic from Night Terror being faster then the eye with at least Supersonic+ combat speed and reactions]. Soul Edge Cervantes also needs weather and electricity manipulation abilities from his ending.
Low Tier = High 8-C [From a very casual Taki. Mass Produces Astaroth should be able to cause this much damage]
Mid Tier = 7-A [I think it should not scale from a full powered Soul Edge since Nightmare main objective is the restore the sword power]
Top Tier = 6-C [Scales to the full powered Soul Edge feat]
God Tier: At least 6-C [Superior to the Full Powered Nightmare and Soul Calibur]
Low Tiers
Xiba: Hyped since his tranings and as Kilik successor, but he should be low tier since is featless and always hungry. Still not ready to face Kilik since he tried to fight an ╬▒ Patroklos that wasn't serious at all.
First Yoshimitsu: Was initially no match for the army nor Nobunaga. Obtained a Soul Edge Fragment, but did not showed anything interesting after.
Second Yoshimitsu: He should be comparable to the first one.
Yun-Seong: Should be compared to other characters of her tier such as Talim.
Viola: Featless, so comparable to the other low tiers.
Voldo: Fought and injured Patroklos, who then fought an Astaroth Clone.
Talim: Has not shown anything that puts her higher, so is the same as Mi-na.
Raphael: Stomped by Nightmare and fought with Cassandra.
Rock: Fought and defeated Aeon Calcos, but was then defeated by Astaroth.
Seong Han: Trained Seong Mi-na and Hon-Yun, he should be as skilled as them.
Seon Mi-na: She got stomped by strong opponents such as Ivy and Edge Master, who briefly trained with.
Natsu: Was trained by Taki, but is the same as Xiba.
Necrid: Same as Viola.
Olcadan: Same as Necrid.
Hilde: I did not found for now heavy material for her being mid tier.
Dampierre: Because he is Dampierre
Amy: Likely trained with Raphael.
Mass Produced Astaroth: Should be all much weaker than the Original, but able to easily destroy a temple.
Cassandra: Fought with Raphael.
Aoen Calcos: Fought and lost with Rock, but over time he became more stronger after he lost his humanity, dominated and became the leader of group of weaker Lizardman's.
Base Pyrrha: Comparable to Patroklos, through she does not want to harm anyone.
Base Patroklos: Initially defeated some fodders, Voldo and an Astaroth Clone, but was easily defeated by Z.W.E.I and Siegfried.
Liexia: The same as Xiba and Natsu.
Hwang: Same as Seon Han and Mi-na.
Mid Tiers
Li Long: Taki herself described him as a incredibly strong. He did not fought regular Cervantes, but him with the Soul Edge.
Original Astaroth: Fought with Sophitia and planned to betray Nightmare and take his Soul Edge, this is all before he was empowered by the shards of Soul Edge.
Zasalamel: Defeated Kilik. He revived the will of the Soul Edge in the form of a new Nightmare with no host, fought Ivy and can fight with Siegried or Nightmare.
Z.W.E.I: Fought and destroyed Graf Dumas Nightmare.
Setsuka: Heavily warned to be too powerful for anyone to battle, fought a Mitsurugi that was not holding back.
Maxi: Managed to block an enraged Kilik, as well as killing the Original Astaroth.
Mitsurugi: Struggled several times with Taki, was excided in presumably fighting Nightmare. Fought Setsuka and tried to fight Algol.
Ivy: Initially assisted Nightmare, then clashed with Zasalamel and Cervantes.
Charade: He is an important part of Nightmare.
Sophitia: Fought with SE Cervantes [Who was not showcasing his full power] and managed to destroy one of his blades, the smaller one. She later fought with Original Astaroth.
Taki: The rival of Mitsurugi, who she struggled with. Dealt the finishing blog that killed SE Cervantes, latter planned to fight Nightmare and was sure that her Mekki-Maru [Her strongest weapon vital in SE and SC] would have destroyed the Soul Edge along with it.
SC Cervantes: Fought with Ivy.
Siegfried: Helped against SE Cervantes and defeated the Incomplete Inferno, which chosed as his vessel.
Not Full Power Nightmare: Released this much energy with the evil seed, requires characters on his tier like Xianghua, Kilik and Z.W.E.I to be stopped.
Xianghua: Defeated Siegfried Nightmare and Incomplete Inferno alsongside Kilik.
Kilik: Defeated Nightmare and Incomplete Inferno alogside Xianghua, tried to fight Zasalamel and harmed a casual Edge Master in order to purify him.
Incomplete Inferno: Fought and lost to Siegfried, Xianghua and Kilik.
Tira: Held back on porpouse against Patroklos in order to continue her plan, unfazed by the possibility of fighting Graf Dumas Nightmare or Z.W.E.I, who she considered pawns of her plan.
Pyrrha ╬®: Forced Graf Dumas Nightmare to escape, then she harmed Z.W.E.I.
Top Tiers
Edge Master: Trained for all his life, knows all the fighting styles and is considered a legend, trained Kilik [who is essential in taking Nightmare] and stated that he'd be willing to fight Algol. [During his training with Kilik in SC3, he was in a casual state].
Full Power Soul Calibur Siegfried: With it Siegfried fought and managed to defeat a Full Power Soul Edge.
Full Power Soul Edge Cervantes/Nightmare/Inferno: Does a feat of this size, the Full Power Soul Calibur is its rival.
Elysium: Comparable to a Full Power Inferno.
Soul Calibur ╬▒ Patroklos: Defeated Pyrrha ╬® with the Soul Edge, then defeated Elysium.
Soul Edge Pyrrha ╬®: Stated to be a better host than Dumas, which allowed to harness the full power of the Soul Edge and fight Soul Calibur ╬▒ Patroklos.
Abyss: A final boss that is born from Zasalamel absorbing the essences of Soul Edge and Soul Calibur.
God Tiers
Algol: The second stronger character in the series, superior to the Complete Soul Edge and Calibur.
Night Terror: The strongest character in the series. Slighty above Algol, but on the same tier.