(This is for the fanfiction "Crossover Together". Info may change from canon information)
Undertaker is a seemingly young, lean man, with long gray hair and a single braid on his right side. Like other Grim Reapers, he has chartreuse phosphorescent eyes. Undertaker has extensive black fingernails and a noticeable scar across his face, neck, and left pinky finger. His robe is predominantly black; additionally, his attire includes an incredibly long top hat, and a gray scarf strapped across his chest and knotted by the hips. He wears an emerald ring on his left index finger; he wears a band of prayer beads around his neck, and he formerly wore a chain with six mourning lockets around his hip. The attire he wears was the typical attire for Victorian England funeral mutes.
When Undertaker was still active as Grim Reaper, he wore a pair of half-framed silver-square glasses, a black trench coat, a white dress shirt, a black tie and a pair of black gloves and shoes. He did not have any scars on his face and neck, but he did have his ear piercings.
Known only by his profession, Undertaker is a mysterious man whose scarred face is rarely fully visible beneath his long hair and crooked top hat. He tends to punctuate his words with sweeping gestures and creepy giggles, and he spends a considerable deal of time inside of coffins. He takes joy in frightening others, as he intentionally acts disconcertingly to provoke a reaction. He has voluminous knowledge in these macabre cases that allows him to be an informant. However, he does not exchange his services for currency, and instead, he expects those that want information from him to amuse him in some way.
While mostly aiding Team Harmony with his information, he doesn't hold much loyalty to one side, as a role of a reaper is played out on Demons, Angels, and Humans alike. Thus, he's mainly neutral and treats other characters mainly the same way, regardless of who they are. This makes him come off as malicious at times, namely with his treatment towards morality as little more than a joke. He usually jokes about death, such as making a recently deceased mod boss "handsome" again, and asking Ciel Phantomhive if it was time for Undertaker to fit him in one of his coffins.
During his time as a Grim Reaper, he was in Elysium when he heard word of Nanael's encounter with Humans. On that night of his visit, he went to see Sombra as a favor to his fellow troops to tell him the news. He joked with Sombra, saying that he wasn't surprised that he would be so upset for the crush he had on her. During the conversation, Undertaker suggested to him to "think outside the box" in regards to the human beings, suggesting for him to go and handle the problem directly despite the rules. After Sombra saw the future of Pluton's destruction, Undertaker joked in saying Tirek was finally showing up, yet Sombra apparently can't do anything about it according to the rules. It was him who gave Sombra the idea to begin his "game" in the first place.
As the years went by, he arrived soon after the Hybrid Train crash had occurred to collect various souls when he came across Alex Armstrong amongst them. He only found it bothersome for how many coffins he had to make, though this was mostly a joke as to how much work he had to do. He began to note the information regarding Edward Elric, Alphonse Elric, and Winry Rockbell, when Alex Armstrong pleaded for him to reconsider. Undertaker warned him that I could cost him, and that even if he wanted to, he couldn't since one of them saw him as the angel he truly was. In the end, intrigued, he revived the three but only after he altered their memories to avoid the trauma the Hybrid Train crash would cause them.
Undertaker, while in his business, was visited by Ciel, Sebastian Michaelis, Nagisa Shiota and Aladdin. They asked him the state of the second mob boss that captured Ciel, but he told them they had to "pay his fee", and amuse him in order to get anywhere. Nagisa and Aladdin tried to do so, but weren't successful, and it only took Sebastian's involvement to get him to talk. He revealed to them that Envy and Lust arrived in his shop earlier to drop off the body, and some information in regards to Homunculus in general.
He was visited again by them (and Pinke Pie) soon after Yugure Kagayaki and Sebastian had gone missing. He again presented his fee to them, and Pinkie Pie successfully managed to get him to laugh. Despite it though, he didn't know what happened to them, but it was just as helpful as it proved that they weren't dead, and if they were he would know about it.
After Hellgar's attack on The Queen, Undertaker arrived on the scene, in glee that he might have to work overtime to make some coffins for the bodies. He revealed to them that Hellgar was dead, saying it had been a long time since he made a canine coffin. He gave his notes to Nagisa, who noted that Twilight and Ciel weren't on the list. Undertaker reassured them that he never misses a body, and there was only one reason for their names to not be there.
Undertaker appears briefly during Class 3-E's trip to Elysium; Undertaker working in the local library as Nagisa and Twilight came to visit. When asked about Underworld information, he hinted at them paying his fee, just for him to laugh and say it was only for his Underworld job, pointing them to a section in regards to demon study. He was later asked about a symbol in one of the library's decorations, and he answered with it being "the Lugia dragon". He explained about his disappearance, and while many thought he was dead, he personally never saw his name in his records.
When Ciel, Sebastian, and Alphonse went on board the Campania, he decided to present himself as well, as a member of the Aurora Society (much to Ciel's confusion). He quickly recognized them out of the crowd despite their disguises, finding it hilarious for Ciel's attempt at the phoenix pose, and when asked of his presence, considered his involvement as professional curiosity, as hospitals have the best customers for him most of the time. When asked for information though, he brought up the proper payment again despite laughing for a minute straight. He "compromised" with ciel and suggested him to give another pose for him, much to Ciel's dismay. He though went away before this could happen once Aleistor Chamber arrived. He was present during the confrontation, but didn't get involved himself and stayed as an observer.
Undertaker reappeared again alongside Aladdin on the Orora High School rooftop, watching Nagisa and Hiromi Shiota go at it for a while in amusement. He confronted Hiromi personally, pointing out how willing she was ready to kill both her son and the Hishimaru leader as a way to tease her. He was mainly present as an observer, finding the whole situation amusing.
Undertaker was present at the meeting held by Kyoya Ootori, and when the full backstory of Sombra was revealed, he was quickly confronted by Team Harmony (Gaara and Edward in particular), his excuse mainly was that he got the ball rolling, but didn't plan on where it would end up. Edward didn't want to leave him scot-free, but Undertaker threatened to have his soul taken back if he were to try anything, only to reveal it to be a joke.
After Ed, Winry, and Nagisa were placed in the Shinigami Realm, Undertaker himself pointed out the idea of their whereabouts. With that in mind, he and Victoria went into the realm, and to Undertaker's original study. He was more than happy to present his work to them, and before he brought them home, gave Ed and Winry's family history along with the end state of the cult group Ciel wanted revenge on. He also gave Nagisa a spell book with Chrysalis's ability page with a note to study on it for the upcoming final battle.