The Judge is a transcended being that determines if a species is worthy of transcendence or if they should be annihilated, and then passes down their fate depending on the test subject's success or failure.
A long time ago, the Judge was born to an ancient alien race, before their transcendence.
Since the dawn of time, when a species reaches a point where they are able to activate the final code on the Anomaly Stone, the Judge is sent to administer a test to determine if that particular species is worthy of transcendence. Should a species fail the test, as many have, the Transcendents would send Gem9 to wipe them out. One such species was the Bardoans. The Judge met with Becca Franko when she discovered the code through the Flame, but Becca refused to take the test and returned to Earth, frightened by what she had seen and feeling that humanity wasn't ready.
In The Flock, Anders reveals that the native Bardoans lost their "last war" to an unknown enemy, resulting in their entire species being turned into crystal via Gem9. This enemy, later revealed to be the Judge and the Transcendents who the Judge represents, is who the Disciples have been training all their lives to fight.
In The Last War, the Judge first appears as Callie Cadogan to Bill Cadogan. The Judge explains who they are and what Bill has to do in order to Transcend. Just after the Judge asks the first question and before Bill can answer, Clarke Griffin appears behind Bill and shoots him, splattering his blood on the Judge's face. The Judge disappears after this.
Sometime later, long enough for Clarke to be annoyed by the Judge's absence, the Judge reappears as Lexa, saying that Lexa was Clarke's greatest love. After Clarke fails to convince the Judge to allow humanity to Transcend, the Judge dooms humanity and disappears to start the process of turning humanity to crystal.
Raven Reyes notices that the Anomaly Stone has now turned red and touches it, on the other side she is greeted by the Judge who is now in Abby Griffin's form. The Judge tells Raven that Clarke doomed humanity due to her atrocities. Raven explains that Clarke did it to save her people. The Judge takes Raven to Bardo to show her Wonkru and the Disciples readying to fight. Raven explains that it was just a distraction. The Judge points to Sheidheda, who proceeds to shoot Levitt, starting a gunfight. The Judge tells Raven it's time to go, Raven asks for more time as Sheidheda gets killed by Indra. Then, Octavia Blake walks out and begs the Disciples and Wonkru not to fight. The Judge stands and watches and is shocked when all of them agree not to fight so they can Transcend. The Judge, per Raven's request begins the process to transcend humanity.
Back on Earth, after chasing Picasso to a beach, Clarke is upset because she'll be all alone. The Judge then appears as Lexa again to Clarke to explain. She tells Clarke that she won't be joining the Transcended, but she won't be alone as her friends decided to return to human form and live their lives with her. The Judge proceeds to explain that the ones who came back cannot bare children and will not join the Transcended when they eventually die. As Clarke looks to her friends, the Judge remarks "a curious species indeed" and disappears.
When greeting their test subjects, the Judge often takes on the personality of the form that they assume for a brief period of time, acting as that person in essence before getting down to the test.
As herself, the Judge is shown to have a stern and unforgiving personality to those who they feel is unworthy of transcendence, remorselessly wiping out other species for failing. However, the Judge does not force a person to take the test as shown by her sending back Becca Franko unharmed after she refused and then not holding her choice against humanity. While they presses a test subject with their questions, the Judge is not cruel with their treatment of them, even when deciding that the species should be exterminated. On a few occasions, the Judge has been shown to apologize for their decisions, but Raven wasn't sure that they actually meant it.
The Judge is shown to have become fascinated with the human race due to what they had observed when Bill Cadogan, Clarke Griffin and Raven Reyes took their test. Initially believing humanity to be beyond saving, despite the beauty they were capable of, they are visibly shocked when their opinion is proven to be incorrect after Octavia Blake talks down the two sides about to go to war, causing the Judge to completely change their mind and allow humanity to transcend. Though they refused to allow Clarke to transcend for killing Cadogan, the Judge takes the time to visit Clarke personally and explain their actions. The Judge came to consider humanity "a curious species" who added much to the transcended beings in the short time they were amongst them and was glad to have been proven wrong. When Clarke and her friends reunite, the Judge is shown to enjoy the pleasure they experience when reunited.
The Judge takes on the form of a test subject's greatest teacher, the source of their greatest failure, or in rare cases, their greatest love. The Judge's true form is unknown.
The Judge: "No. This is who you chose. We most often take the form of the subject's greatest teacher or the source of their greatest failure. In the rare emotion-driven species such as your own, it can be their greatest love. Seems you're faced with all of the above."
The Judge: "Excellent. Here's how it works. We talk. I ask questions, you answer truthfully, if you do not I will know it. If your answers are satisfactory, your consciousness and that of your entire species will become one with ours. You will transcend your human form and then become infinite. If you fail to earn this evolutionary leap, you will be eliminated, turned to crystal the way the Bardoans and countless other civilizations before them were. Sound good? (Cadogan nods) The first question is about love. So few species are given this gift, yet you have attempted to erase it in pursuit of this moment. Why?" (Clarke kills Cadogan)
Clarke Griffin: "I know. (releases the Judge after a few moments) The test isn't over?"
The Judge: "No. I'm sorry. We can't stop what's been set in motion. Your species must now be judged through you. (walks over to Cadogan's pool of blood) This man was unarmed, no threat to you, yet you killed him. Why?"
The Judge: "You suffer and inflict suffering on others. Pain begets pain. That's not justice, Clarke. You say you do these things to protect your people, but you're all one... people. That much Cadogan had right."
Clarke Griffin: "He created a world without love to get to you. He killed my child to get to you. Have you ever considered that you're the problem? How dare you judge me when you annihilate entire species because they don't live up to your ideals? Have I pulled the lever to commit genocide? Yes, I have. And did love make me do this? You're damn right it did! But what's your excuse? You play games with people and call yourselves higher beings. You are no better than the murderers I've killed, and you are no better than me!"
The Judge: "I am sorry for all you have lost. You've borne so much... but if you are humanity, then I'm afraid humanity is not worthy of taking the next step. It has been decided. (in Trigedasleng) Your fight is over."
Raven Reyes: "Are you? Because to me it looks like you don't feel anything. Have we made mistakes? Yes. Clarke, me, all of us. But we were just trying to survive. I'm not saying we're ready now, and if you don't want us to join you, fine... but at least let us live, keep trying to do better. We will. We have."
The Judge: "You say you have, yet even now you are poised and on the brink of self-extermination. Look. (shows Raven the standoff between Wonkru and the Disciples) You say you're trying to do better, but all I see are two tribes of frightened creatures willing to kill the other to save themselves."
Raven Reyes: "But they're not fighting, it was just a distraction. This proves nothing."
The Judge: "They will fight, Raven, like they always do."
Raven Reyes: "You can't possibly know that." (the Judge draws Raven's attention to Shiedheda)
The Judge: "Don't you see? Despite the beauty that humans are capable of, you can't break free from the cycle of violence. This is who you are. That's why you failed the test. It's time for us to go, Raven. The end of the human race is here."
Octavia Blake: "Enough! What the hell are we doing here? You swore an oath to fight for all mankind. Well look around you! We are mankind, we are Wonkru! If I kill you, I kill myself. If we keep killing each other, there won't be anyone left to save. Our fight is over, Indra."
Octavia Blake: "We're unarmed. I know you're afraid to walk away from everything you've spent your lives training for. I am too. My brother believed as you do.. that transcendence -- whatever that means -- is within reach. He died for that belief and for that he'll never get there. Never transcend. I don't know what I believe, but I do know that if we fight this war we don't deserve to find out if he was right. We don't deserve to survive. We fail, we die."
The Judge: "In a manner of speaking, yes. Her consciousness has joined ours. She's at peace. She'll never feel pain, she'll never die. She knew that living here without anyone her own age to love is something you wouldn't have wanted for her even if it meant you being alone. I suppose that choice was made easier with the realization that you wouldn't be."
Clarke Griffin: "Transcendence is a choice? You can choose to come back?"
The Judge: "Of course. Though until now no one ever had. (Clarke sees her friends nearby) There won't be offspring and they won't join us when they die. None of them seem to care."
The Judge, in the form of Lexa, has the last scripted line of the series, commenting on how humanity is a curious species.
Having stated to have wiped out countless species, it can be assumed that the Judge has the biggest kill count by far of any character in the series.
Given that the Judge stated that they and Raven were meeting 42 billion light-years away from Earth, their home, or at least wherever they meets her test subjects, is likely in another galaxy entirely.
Amongst the knowledge that the Judge can assimilate from the test subject appears to be other languages. While talking with Clarke in Lexa's form, the Judge was able to speak in Trigedasleng.
The Judge was originally thought of as the primary villain of the season, and the secret enemy that humanity would be facing in the "Last War". However, this was proved to be incorrect.