(This is for the fanfiction "Crossover Together". Info may change from canon information)
Sebastian holds many traits as typical to a standard butler: willing to follow any orders given to him by his master, and not showing much anger or frustration towards his master's needs. He doesn't hold too much remorse towards the safety of others, Ciel included, as many of his reactions are usually calm and collective. He is also shown to be very clever, even able to trick Sombra himself.
Although he is a loyal butler, he still does make his own decisions, namely if he could see an advantage his master can gain, or if it can better reach his goals, sometimes being done without Ciel's knowledge, as evident when he joined Sombra without telling Ciel about it to gain information on the missing persons case.
Upon his first meeting with Ciel, he reveals himself to him as a large crow rather than his demonic self. He does something similarly when he disguised himself as King Bradley in order to flush out Envy.
Enhanced physical abilitiesEdit
As a high-level demon, Sebastian has a naturally stronger amount of strength and agility when compared to others, including other lesser demons. A notable example is when he caught up with the train heading to Central on foot, while dragging Alphonse Elric and Edward Elric behind him with relative ease.
Sebastian is able to use the Faustian Contract on humans, allowing him to feed on their souls once his side of the deal is made.
This ability is mostly hinted on, as Sebastian was able to sense the "holy energy" of Nagisa Shiota as he and Ciel passed him up.
During his youth, Sebastian came across Ciel Phantomhive while he was being tortured, and decided to offer him a contract in order to help him escape. When not given an answer the first time, Sebastian left him with a spell to summon him whenever he was ready. Once Ciel did that, he once again offered him the contract, to which Ciel accepted. He did as he offered and managed to rescue Ciel, and for the years following has become a close companion and butler to the Phantomhive Manor.
During a trip to the local market, Sebastian started to take first note of Team Harmony as he and Ciel were heading home, and informed him about the "holy energy" they had on them. Ciel, curious about them, got Sebastian to track them down and invited them to the manor. Her was then put in charge of the preparations alongside Bardroy, Mey-Rin, and Grell Sutcliff, of which he did the most efficient work of all of them, even doing some of their work himself after finding them messing up their tasks. The visit processed according to plan, but when he found his master kidnapped by Envy, he was the first to go off after him, being joined later by Team Harmony. He made short work of the guards, defeating multiple men during the rescue, and later on easily overpowered Envy himself. Shortly after, he reveals himself to Nagisa Shiota of his demon status, and even asks if he was indeed going to kill him, which Nagisa declines.
Not long after word of Twilight's disappearance, Sebastian was found by Envy and Lust, Lust whom tried to convince him to join Sombra instead of wasting his time as a butler. He declines at first, but after some time, he decided to join Sombra personally, though mainly in order to figure out the missing persons case within England. During his stay, he took the role of being Sombra's personal servant, despite being told he didn't have to do that anymore, and eventually revealed to Sombra of his Faustian Contract, and attachment to Ciel, claiming that he didn't want his meal to be medaled with.
After gaining enough information to satisfy, Sebastian left Sombra and returned to the Phantomhive Manor to find the servants fighting Lust and Envy. He helped clean up the bloody mess when they were done, and figured out that his master was with The Queen. He arrived just in time to rescue Ciel and Twilight from Hellgar.
His role in the third season has been downplayed to a loyal butler to Ciel, often seen with his master and following what orders he's given. He did take part though during the Pluton attack by disabling Pluton's wind abilities alongside Edward Elric.
As with season three, season four also had Sebastian in a more of a supporting role.
His most notable role in Season five comes into play when he, Alphonse, and Ciel went after Aleistor Chamber and the Aurora Society boarding the Campania. Not too long after the society's demonstration of their revival work, Sebastian was tasked by Ciel to check in on Margaret Connor. It was during this though that Sombra had found him, and presented Sebastian with a "loophole" around the Faustian contract with a method to extract Ciel's soul without killing him. Sebastian then warns Sombra that he will die if he were to take his soul before the contract were to complete, and he decides to make Sombra a deal, with Ciel as a reward if he agreed to it. When confronting Aleistor, Sebastian had the time to fight off the many revived humans, up to when Sombra arrived to complete the deal, and take Ciel Phantomhive away, but not before Sebastian extracted Ciel's soul. Upon doing so, Sebastian consumed it right in front of Ciel, resulting inSebastian poisoning himself, and dying shortly afterwards.
His opinion on Ciel is a bit mixed, and it's difficult to say if he truly cares for Ciel or not, as much of his protection towards him is mainly because he didn't want anyone stealing his meal away from him.