Kelvin is completely wrapped in bandages with only his left eye and mouth revealed. He dresses in a formal suit, which befits his position as a baron. Additionally, he is wheel-chair bound because he is missing both legs, after Doctor had implemented surgery on him.
Before his plastic surgery, Kelvin wore glasses, had hair, and could walk. His undue obsession with "Ciel Phantomhive", as well as his excessive desire to become more beautiful for him, eventually impelled him to drastically modify his appearance.
In the past, Kelvin was well known for his philanthropic activities. He welcomed children into his home and helped them survive when they had been abandoned, abused or otherwise neglected by their parents. To his superiors, especially Vincent Phantomhive, he was timid, meek and humble, but he highly respected them.
However, his kindness and generosity was affected significantly when he encountered "Ciel Phantomhive" and consequently developed an inordinate obsession for him. Currently, Kelvin presents himself as a vulgar, depraved, and perverted individual with an uninhibited interest in "Ciel." Moreover, he is rather oblivious to how his corrupted thought processes appear to others, as he seems surprised when "Ciel" articulates his aversion to Kelvin's speech and actions. He holds a blatant disregard for all children except for "Ciel"; in fact, he deems it amusing for children to die gruesomely, and strives to recreate the day "Ciel" was sacrificed. He also expends children as materials for Doctor's experiments with making artificial limbs.
Kelvin uses the children in the Renbourn Workhouse as hostages to control the first-tier members and get them to do his bidding, as unsavory as they may find it. Later, Ciel Phantomhive and Sebastian Michaelis discover that Kelvin had lied about keeping the other children at the Renbourn Workhouse alive and that he most likely allowed Doctor to kill them for his experiments with artificial limbs. However, this did not stop him from continuing to pretend they were alive so he could maintain his influence over the circus members, particularly Joker.
Five years ago, Kelvin met Vincent Phantomhive and his sons, "Ciel" and Ciel, at a party. He came to view them as special and beautiful—especially "Ciel". After desperately researching the Phantomhives, Kelvin unearthed the "Evil Noblemen," and concluded that only special people could approach other special people, when Diedrich had led Vincent away. Kelvin decided to undergo numerous plastic surgeries in order to become more beautiful to suit "Ciel". His wife became disgusted with his obsession and left him, but he did not care as long as he could get closer to "Ciel" as a result.
When he received news that the Phantomhive family had been killed, Kelvin was very upset and stated that his life had no purpose anymore. Some time later, he acquired knowledge that "Ciel" is alive and was to be used as a sacrifice. However, he could not attend the ritual sacrificing, as he had recently undergone another surgery and was forced to remain bedridden for a month.
When he learned that "Ciel"—who to Kelvin's lack of knowledge was, in fact, his twin, Ciel, who had usurped his identity—had survived the sacrificing and had slaughtered all of his attackers, Kelvin had the Noah's Ark Circus kidnap children in an attempt to lure out Ciel; in this, he succeeds.
The Noah's Ark Circus first-tier members refer to Kelvin as Father, as he is the one that finances their circus. Ciel Phantomhive orders Sebastian Michaelis to find out his real name, and after seducing Beast, he learns that Father is actually Baron Kelvin.
Kelvin is surrounded by lifeless children, when Joker arrives at his manor. He is excited by Joker's appearance and inquires if "he" is with him, referring to Ciel Phantomhive. Joker says that he is not and that he believes the circus may have been marked by Scotland Yard. Kelvin is convinced that the newcomers who invaded their tents must have been Ciel and his butler, and orders Joker to prepare a feast for their arrival. When Joker hesitates, Kelvin gets angry and reminds Joker that he is in control of the fate of Joker's brothers and sisters back at the Renbourn Workhouse; this compels Joker into doing what Kelvin wishes. Satisfied, Kelvin asks Joker to take a picture with him, declaring that he wants a new picture of a "devoted son and happy father."
Subsequently, Ciel and Sebastian enter the mansion of Kelvin, who they learn has been waiting for them. After witnessing a grisly and horrific circus performance, put on by untrained, brainwashed children, which Kelvin finds hilarious, Ciel pulls a gun on Kelvin; Joker rushes to his rescue, but Kelvin refuses his aid as it may harm Ciel. Ciel then requests to see the other kidnapped children. Kelvin eagerly complies, leading Ciel to his basement, and states that he has something he would like to show Ciel anyway.
Kelvin has remade the basement into the room Ciel was sacrificed in, and even has children locked in cages. He discloses that he has been obsessed with Ciel for five years, and states his wish to reenact the day Ciel was sacrificed. An appalled Ciel shoots him, and Kelvin cries out of pain and because of the fact that he is unable to experience the "beautiful" death that the ones who sacrificed Ciel experienced. When Joker rushes to kill Ciel, Sebastian severs his right hand. Ciel tells Kelvin if he wishes to die like the people who witnessed his sacrifice, he should beg his demon (i.e. Sebastian) to do it.
After Doctor goes in the room and reveals that he is just as sadistic and twisted as Kelvin is, Ciel orders Sebastian to kill them both, which Sebastian does by dealing Doctor a fatal blow and stomping on Kelvin's head. Sebastian then sets the estate on fire, and they leave the bodies there to burn.
- (To Joker) "Isn't it true? I raised you to be a good child. You do want your siblings still at the Workhouse to be raised into splendid adults also, right? The good little Joker will listen to what Father says, right?"
- (Referring to Ciel, Vincent, and "Ciel" Phantomhive) "I just couldn't take my eyes off of those three. In the dark night they were like a silently rising moon floating in the sky. I understood at the moment that these people were special people. Since then I desperately researched the Phantomhive family. I wanted to know exactly why they were so special. Then I knew. The true face of evil noblemen. Underneath beautiful flowers there were thorns. I couldn't help falling in love with what was under the rose."
- (Referring to the Phantomhive family) "Special people can only be touched by other special people. I'm too ugly to even think about touching them. My wish to be wrapped in that velvety world of darkness would never be granted."
- (Referring to "Ciel Phantomhive") "He will also be wrapped up in beautiful thorns and become an evil flower, won't he? In that world of darkness would he become the only great cold moon? Completely out of reach. No! I want to touch him. I want to be a special person."
- According to the official Kuroshitsuji character popularity poll, Kelvin is the forty-first most popular character in the series, with 6 votes. He shares this spot with Maurice Cole, who also received 6 votes.
- Kelvin is frequently associated with the nursery rhyme Tom, Tom, the Piper's Son, as he signs his name in a letter to Joker as "Tom the Piper's Son" and has the lyrics to the rhyme relayed in several scenes relating to him.