After being name-dropped quite a bit in previous episodes, fans were officially introduced to Arrow's Joe Wilson.
The son of Slade Wilson/Deathstroke (Manu Bennett), Joe had been missing in action for quite some time. But with the help of Oliver Queen (Stephen Amell), Slade was able to locate Joe -- and was surprised by what he found.
So who is Joe Wilson, anyway? Well, his comic canon is pretty complex. But here's an abridged version...
In the comics, Joe was the youngest son of Slade and Adeline Kane, a name already mentioned in tonight's episode. As a child, he was held hostage by Jackal, a terrorist wanting to draw information out of Slade. Slade initially refused to give information, causing a terrorist to slit Joe's throat, rendering him mute.
Adeline divorced Slade and raised Joe, with the boy learning to communicate with American Sign Language. Joe soon discovered that he had metahuman powers, specifically the ability to possess a human simply by making eye contact with them.
As a young adult, Joe and Adeline learned about Slade's contract on the Teen Titans. Joe adopted the superhero name of Jericho, and joined Dick Grayson to help save the group.
The rescue mission was successful, although the Titans were apprehensive about Joe's alliances with Slade.
Joe was later possessed by the souls of Azarath, which had ties to the father of fellow Teen Titan Raven. He fought to obtain individual vessels for the souls, and used current and former Titans members to do the job. Along the way, Joe got new powers -- namely a healing ability, which allowed him to speak again.
Joe was ultimately confronted by Nightwing, Troia, and other allies. The real Joe ultimately resurfaced, and begged for his father to kill him. Slade followed through with it, as a way to spare Joe from more pain.
Years later, it was revealed that Joe's spirit had been hiding dormant inside of Slade's body. It was awoken by the death of Donna Troy, with Joe now hell-bent on disbanding the Titans for their own safety.
He fought with the Titans repeatedly, with his soul jumping from body to body. Raven ultimately absorbed his essence, and transferred it to a computer file stored in Titans Tower.
Raven later helped resurrect Joe into a new body, allowing him to rejoin the Titans. He bonded with the team, particularly his half-sister Rose Wilson/Ravager.
The New 52 reboot actually included multiple versions of Joe, which has drastically different impacts on the DC canon.
The first incarnation of Joe had been presumed dead as a child, after North Koreans attacked Slade's home. But Joe returned in his early twenties, with metahuman powers and a vendetta against his father.
This version was later retconned, with Joe re-established as a more heroic character. Jericho was kidnapped by his grandfather, Charles Wilson/Odysseus, who wanted to experiment with Joe's powers. Joe was ultimately saved by his family, but he was left with a severe inability to control his own powers.
Joe ultimately trained under Ra's al Ghul, who helped him gain control over his powers and help save his family.
In the newest DC Comics continuity, Joe maintained his original origins, as well as his muteness. He also was established as canonically bisexual, something fans had speculated about for quite some time.
Joe grows up to be a higher-up at a tech firm, developing a new bit of technology that allows him to vocalize his thoughts. He is engaged to his sign language interpreter, but the wedding is threatened by David Isherwood/Doctor Ikon, a former lover of Joe's. Joe possessed and appeared to kill David, motivating him to become a superhero with the help of the Ikon suit.