Warning: SPOILERS for Lucifer Season 6.
Lucifer ended with Lucifer Morningstar (Tom Ellis) becoming Hell's therapist but this leaves the fate of his evil twin brother Michael (also played by Tom Ellis) as an unanswered question. Michael was the main antagonist of Lucifer season 5 but in season 6, the Devil is primarily concerned with getting to know his estranged, time-traveling daughter, Rory Morningstar (Brianna Hildebrand), and solving the mystery of why he vanishes from her life.
Michael was the worst of Lucifer's enemies and he very nearly achieved his goal of becoming God in Lucifer season 5. Initially posing as the Devil, Michael sowed discord within Lucifer's inner circle and he not only kidnapped Chloe Decker (Lauren German) but also manipulated Dan Espinoza (Kevin Alejandro) into shooting Lucifer after revealing that he's the Devil. Michael stayed in the shadows when God (Dennis Haysbert) came to Earth in mortal form but it was so that the twisted archangel could gather support in Heaven for his bid to become the new God. Lucifer and Michael led opposing factions in a war for God's throne and Michael killed Chloe, which forced Lucifer to go to Heaven to bring his beloved detective back to life. Ultimately, the Devil triumphed and earned the right to become God (although Lucifer didn't go through with it). Lucifer cut off Michael's wings as punishment for all of the sorrow his identical brother inflicted.
Lucifer season 6 revealed that the wingless and humbled Michael was banished to Hell and that his damnation involves him being powerless and cleaning the underworld's gutters. Rory briefly saw her uncle in Hell but stopped short of speaking to him about how to destroy Lucifer. But by the end of Lucifer season 6, Rory and Lucifer made amends and the Devil learned that he abandoned his daughter throughout her life because Lucifer chose to return to Hell and become the underworld's therapist. Instead of becoming God, a destiny embraced by Amenadiel (D.B. Woodside), Lucifer chose to do for the damned what Dr. Linda Martin (Rachael Harris) did for him and help everyone in Hell break their Hell loops so they could enter Heaven. But this also means that, at some point, Michael would have to find himself on Lucifer's couch as well.
To fulfill his self-appointed destiny, Lucifer (and Chloe, who joined him as the Devil's partner in Hell after she died) will have to offer Michael the same counsel he is giving the rest of the damned souls. This means Lucifer will have to not only face Michael once more but show his evil twin more patience and understanding than he was ever able to muster before. Alternately, has Michael changed from being a villain after his defeat, humiliation, and his penance in Hell?
Lucifer and Michael have been like oil and water since the dawn of time. Whether or not Lucifer can actually offer Michael effective counsel to break his Hell loop so Michael can go to Heaven will be the biggest test of Lucifer's skills as a therapist. But the fact that Lucifer will have to deal with Michael again also shows how Lucifer, typically, didn't think his choice to become Hell's therapist through. As with most of the Devil's choices, Lucifer made an impulsive decision, albeit out of love for Rory and to protect his daughter's timeline. If Michael has actually changed, and if the Devil can actually help his twin brother, it would behoove Lucifer to send Michael back to Heaven, just so he becomes Amenadiel's problem as God if Michael reverts back to his nefarious ways.
Unfortunately, since Lucifer has definitively ended on Netflix, the question of whether Michael can change and go back to Heaven won't be answered unless there's some sort of continuation of the story. Michael basically became an afterthought after he was so prominent (arguably to a fault) in Lucifer season 5, and he was only briefly seen from behind (and apparently not played by Tom Ellis) in Lucifer season 6. Lucifer's decision to return to Hell and save all of the damned souls was a noble ending for him, powered by the belief that "if the Devil can be redeemed then anyone can." But sadly, fans will never know if Michael's redemption will happen and how Lucifer will save his evil twin brother.
Lucifer Season 6 is available to stream on Netflix.