Over his years of living, Merlin has met many people. With some he has had a positive relationship with while with others has been negative.
Occult Research ClubEdit
Merlin and Issei have a good teacher-and-student relationship. During his time as a teacher, Merlin was the only teacher that did not hold a dislike towards Issei because of his perverted acts and would try to help him with his schoolwork. Merlin also said that Issei was similar to Arthur: both did not take things seriously but held great potential, as well as having wielded the same Sacred Gear, Boosted Gear. Issei in turn admires Merlin even after discovering his identity, and wishes to live up to his expectations. Despite having dropped his alias, Melvin Amber, Issei still refers Merlin as "Amber-sensei".
Koneko has a father-daughter relationship with Merlin. When he was under his disguise as Melvin Amber, he would always give Koneko treats or help her out with her schoolwork. Koneko in return would always help Merlin tidy up after school. She also described Merlin as having a scent that reminded her of nature during the seasons, most likely due to Merlin being a Druid and gives off a kind and helpful aura. Even after leaving the school and revealing his identity, Merlin and Koneko are still rather close. He was the one who helped Koneko out of her fear of Senjutsu and was the one who brought her and Raiden together, which was a positive thing for both of them.
Four Great SatansEdit
The relationship between Merlin and Sirzechs is complicated. Sirzechs first met Merlin when he was searching for him for Merlin's help against the Old Satan Faction. Merlin found Sirzechs annoying and arrogant, the former for the Devil's constantly pursuing him for his help, and the latter because Sirzechs once challenged Merlin to a duel for his coporation, which Merlin soundly won. Afterwards Merlin found Sirzechs annoying, since the now Maou would always look for him in order spend time together. During those times Sirzechs would either brag about his family (especialy his son and sister) or try to recruit Merlin into his peerage. Usually Merlin would just leave SIrzechs in the middle of nowhere, knock him out or both. Despite that Merlin respects Sirzechs for his great prowess and his care for his people and family. Sirzechs considers Merlin a friend and respects his incredible magical talents and his achievements.
However, their relationship became strained when Merlin learned that Sirzechs ordered the extermination of the Nekomata race after Kuroka killed her master. After kiling the extermination parties, Merlin confronted Sirzechs and beated the Devil within an inch of his life, leaving a large scar on the redhead's body. Though their relationship has improved somewhat, Merlin still gets annoyed by Sirzechs' antics and would usually greet him with a hit on the head. Still Merlin was nice enough to agree with Sirzechs' request and train Rias and her peerage for their Rating Game.
Serafall fell in love with Merlin when they first met when she, Sirzechs and Ajuka were searching for him for help against the Old Satan Faction. Every time she gets the chance, Serafall would pester Merlin to join her peerage and would get jealous whenever another women would talk with Merlin. While he does not share the same feelings as her, Merlin has recognized Serafall as a powerful Devil and is impressed with her skills in Ice Magic. Though, he does find her constant attempts to recruit him slightly annoying and would always teleport whenever Serafall tried to hug him.
God of the BibleEdit
Merlin and Dulio appear to get along very well with each other. Merlin mentioned that Dulio was the only Exorcists besides Vasco Strada that he could get along with. To the magician, Dulio is the kindest soul he had ever met, second only to Asia. Dulio finds Merlin fun to hang out with and they usually talk about the kinds of food in the past whenever they meet.
Merlin appears to be the only male whom Rassei likes, as he never shocks the magician. This may be because that, just like Asia, Merlin also possesses a high Dragon compatibility and/or he has merged with Kur. Merlin also seems to like the young Sprite Dragon, as he gave Asia a book about dragons in order to increase Rassei's strenght and even unlocked a portion of his powers.
Out of all the members of Team Vali, Merlin interacts more with Arthur, second only to Le Fay. Upon first noticing him, Merlin had mistaken Arthur as his ancestor and called him "your majesty". He later gave Arthur Excalibur Ruler once the young swordsman proved himself worthy of wielding it. Merlin sees much of King Arthur in his descendant, going as far as to say Arthur could become the leader of his own group if he so desired. The great magician even helped Arthur with his family by taken in Arthur's lover, Elaine Westcott, as his student.
Arthur also holds Merlin in high regards. He enjoyed speaking with Merlin, hearing the tales of his ancestor and was even willing to learn a bit of magic from the great magician. When Merlin requested the return of Excalibur Ruler, so he could reforge Excalibur and return it to the Lady of the Lake, Arthur agreed without hesitation. Arthur also felt indebted when Merlin took both his little sister, Le Fay, and his lover, Elaine, as his students, saving the latter from his family's punishment.
Le Fay PendragonEdit
The young magician is propbably the only member that interacts the most with Merlin. When she first discovered who he was, Le Fay fainted from excitement at meeting the legendary wizard. She was also amazed at the amount of spells that Merlin possessed during his mock battle with Team Vali. Le Fay holds the grimoire that Merlin gave her as her greatest gift, and has even started to learn Druid Magic, Merlin's primary magic. She hopes to become a Druid just like Merlin and said she was honored when Merlin asked if she would like to become his student. Le Fay strongly believes that Merlin is the greatest magician in the past, present and future amongst all supernaturals and humans. It is hinted that she may have developed a crush for Merlin.
In return Merlin also holds high standards for Le Fay as her progress as a magician. He was greatly impressed with her experience in spells during the mock battle between Merlin and Team Vali, saying she was the first skilled magician Merlin had met in over a century. He even gave Le Fay one of his grimoires and spoke many times with her on the subject of magic. Later on Merlin took Le Fay in as his student, believing that she has the potential to surpass her ancestor, Morgana.