The Plot Driving Theory
The basis of this theory is that the Black Knight is simply a token character who’s purpose is to drive the story arc forward. Admitably, this is simply the most plausable theory, the biggest proof of such being that the game never explains his identity or back story. Obviously, this isn’t very popular with fans.
There’s also the matter of the sheer amount of dialogue from various characters placing importance on the Black Knight’s true identity. Caineghis’s discussion with Ike on the matter in chapter 9, for example, is one example of this. Furthermore, the scene where Greil and the Black Knight fight explains quite clearly that Greil knew exactly who the Black Knight was. Unfortunantly for us, he decided to take it to his grave.
This, coupled with Sephiran’s monologue during the ending, lead many people to believe that Fire Emblem 10 will be a direct sequel to Path of Radiance (backed by the fact that the Fire Emblem series generally devotes two games to each universe, and Path of Radiance is the only one thus far in its timeline). If there is a sequel, the Black Knight’s identity will likely be explained therein.
The Zelgius Theory
This theory holds that the Begnion general, Zelgius, is the Black Knight. Zelgius is the general who leads the reinforcements who come to Ike’s aid at the Begnion Senate’s bidding, arriving around the time Ike begins to march out of Daein. The Zelgius theory is mainly based upon the resemblance of growth rates and other data embedded in the game. In the game’s code, Zelgius has character data, just like other characters, despite the fact that he never appears in battle or on the map (something that is not uncommon in the series). If you compare his data with the Black Knight, you’ll find much of it is identical and others very close.
Their growth rates are exactly the same:
Having the same growth rates is exceedingly rare. However, to be fair, it’s not the only occurance of such in this game. The only other occurrence of it in this game is Ashnard and Petrine (possibly because they both can be obtained on the Trial Maps) and Oliver/Hetzel/Lekain (Oliver has growth rates for the Trial Maps, the latter two only appear in dialogue and can be logically considered clones of Oliver).
On the other hand, the growth rates may be the result of simply copying and pasting, or the like. The Data for Zelgius and the Black Knight are right next to each other. Since NPC character’s growth rates aren’t exactly necessary, the may have simply carried them over. Sephiran’s data, though, is also right next to the other two, which may strengthen the theory in the eyes of some.
Also of note is that neither their stat additions nor base stats are identical. However, both the stat additions and base stats are, admittedly, quite similar. The only real difference is luck. Hypothetically, one could say that perhaps goddess-blessed armor severely drops your luck, or possibly that using blessed armors in corrupted ways drops luck.
From a story stand point, Zelgius is a Begnion general and quite a famous one at that. On the other hand, the Black Knight is a mysterious and feared figure working for Daein. It’s hard to see how they can be the same person. Yet, most importantly, Zelgius and the Black Knight never appear at the same time (keep in mind that a lot of characters don’t, though). His warp powder would allow him to live this “double life” with no one the wiser.
Another point that some add to this is that when the Black Knight is fighting Ranulf. Sephiran immediately stops the Black Knight, which rises the question of what kind of respect the Black Knight has towards Sephiran. If Zelgius is the Black Knight, both he and Sephiran work for Begnion so of course he would listen to Sephiran’s persuasion (some have taken it a step further to say that Sephiran may be a traitor as well). If the Black Knight is working for Begnion it makes it far easier for him to obtain the goddess-blessed armour for both him and Ashnard. He could have stolen them, but how did he know where to find them? Again, perhaps Sephiran have played a part in this.
On the other hand, it is also perfectly logical that the Black Knight stood down simply because he didn’t want Begnion to get involved at that point in the war. There’s also the problem that Zelgius’ presence in the story is very minimal. Though to some, this gives rise to the question why the makers of the game included him at all, he does play the specific part of representing the Begnion reinforcements from a story standpoint, which forces Ike and the Greil Mercenaries to evaluate their overall goal and strategy — a minor plot point in the game. Another major road block is the fact that even after the Black Knight dies, Zelgius leads the Begnion army during the final battle. The only way around this is if Ike/Nasir only thinks they killed Black Knight.
The Greil’s Eldest Son Theory
In this theory, the Black Knight is Greil’s son (Ike’s older brother). He may want to kill Greil because Greil killed his mother.
Those who hold to this theory assert that the Black Knight shows little true desire to defeat Ike. He appears on the map in two chapters, only to stand by and do nothing if he is not attacked. He even gives Ike the chance to let him defeat him! Some have interpreted this as the Black Knight sharing a brotherly bond with his younger brother. This does little — if anything — to expound on the encounter of the Black Knight with Sephiran though.
The question also arises whether Greil would actually fight his own son. His morals would possibly not allow that. His love for Elena would probably lead him to think that it is right for his son to kill him. Then again, he has Ike and Mist to look after, still.
One of the most glaring problems, however, is Greil’s recognition of the Black Knight. A vague, sudden “I remember you,” seems a little awkward. If the Black Knight is Greil’s son, Greil shows very little emotional reaction.
Aside from that, if Greil had another son, others should have known as well. Greil was a very famous character. If it were his son, it was a lost or secret one — which, considering many of the other characters and dialogue, seems a bit far-fetched.
An alternate version of this theory is that the Black Knight is not Greil’s son, but instead, his brother. Obviously, this carries the same evidence and inherent problems.
The Branded Theory
It could be that the Black Knight is “branded,” which means, like some of the other characters in the game, he is half Beorc and half Laguz. In this case, it is likely that one of his parents is a dragon laguz. If he was born in Goldoa, he could have been banished from there because he was neither beorc or laguz. It is also possible that he took the blessed armor with him from there. If the beorc parent was from Daein, that could also explain how he came under training of Gawain there during his banishment.
It’s also argued by a few that that teleportation powder would be something for dragons only, and only they have the strength to use it (as the Black Knight mentions that it takes a lot of energy). Being big and slow as they are, teleportation powder seems like it would be a deadly tool if used by dragons.
Those who hold to this theory point to the fact that the Black Knight doesn’t actually kill Ena, a dragon, while he does hurt at least one bird laguz and probably would have killed Ranulf if he wasn’t stopped. He is also described as being a fierce warrior in one of the battles near the end of the game where the hawks played a major part — Leading one to believe that he slaughtered his fair share there as well. On the other hand, it could also be argued that Ena lived simply because Ike happened to run into the room at that exact moment, catching the Black Knight off guard. Theoritically speaking, he could have also just naturally missed, although one has to admit, something like that is rare in this type of fiction.
The Brothers-in-Arms Theory
This theory stems from the encounter between Sephiran and the Black Knight. The Black Knight “obeys” the prime minister of Begnion, Sephiran, when he tells the Black Knight not to kill Ranulf, even though they are on different sides of the war.
Some hold this as evidence that both Sephiran and the Black Knight are involved in something that serves a higher purpose in both their visions. They may have been entrusted duties to carry out. Depending on how you interpet the scene, the encounter suggests a rank order, since the Black Knight has no obvious reason to do what Sephiran says, yet he obeys him. It’s possible that they are involved in a cult which serves this higher purpose (for good or evil) and where Sephiran outranks the Black Knight.
However, a lot of people respond that this is simply a misinterpetation of the scene that plays out. The Black Knight could also just as easily have stood down because he knew or believed Sephiran to be a match for his power, or because he didn’t want to draw Begnion into the war yet.
The Fake Sephiran Theory
More a side note than a Black Knight theory, but worthy of note since most Black Knight theories involve his encounter with Sephiran, the prime minister of Begnion.
Sephiran’s assorted information is embedded in the data on the GCN disc as “fake_cepharan” which could imply that Sephiran is an imposter. This leaves a lot of room open for speculation. If Sephiran is fake, the Black Knight could even be the real Sephiran, or perhaps the “fake” Sephiran Ike meets the first time is a look-alike, different from the one he talks to at the end?
The problem here is that it’s never actually brought up in the game, and since making a game is a long, drawn-out process stretching over a year or more, many things change and evolve as time goes on. The “fake_cepharan” could simply refer to a discarded sub-plot, or even a character that doesn’t exist anymore (one can find quite a few “left overs” looking through the raw data on the disc). It could also be an in-joke. The possibilities are endless.