Crona's weapon, as well as part of their blood. When Crona was a child, Medusa melted down Ragnarok, at that time a regular weapon, and combined him with black blood. She then injected this mixture into Crona's own blood, creating an unwilling symbiosis and permanently fusing Crona with Ragnarok.
Ragnarok is a playful character, and acts like an annoying older brother to Crona. He often gives the meister noogies, steals their food, plays with their nose, and even lifted up Crona's robe at one point, almost exposing the underwear. Whenever Ragnarok is angry with his meister, he will not hesitate to pop out of them and begin attacking them in an annoying, playful, but slightly violent way. He also uses Crona as a human shield at times, notably only for non-lethal attacks, like Maka Chop.
However, despite his teasing, Ragnarok does seem to care about Crona. He heals Crona whenever needed (although always pestering for a "thank you very much" afterword) and provides constant vocal motivation during battle. Ragnarok realizes that if Crona dies, he will die too, ultimately driving him to protect his meister as much as possible.
Crona's first friend; they share a similar soul wavelength. It was during the underground the DWMA battle in which the Kishin was revived where Maka and Crona became friends: after seeing into Crona's soul and realizing the emptiness and sadness of their past, Maka came to understand Crona on a deep level. Not long after, Maka gave Crona presumably their first loving physical contact when she hugged them and gave them a handshake.
After the underground battle, Maka and Crona become formidable friends. It is with Maka's help that the meister learns how to fit into the DWMA. At first, Crona won't even leave a room without Maka, though they become more open later on. Although the strong relationship seems to be one-sided at times (as Maka has many friends and often does not spend time on-screen with Crona) ultimately Maka's love for Crona is shown in the Anime when she decides to help Crona defeat Medusa rather than help the DWMA bring down the Kishin at Baba Yaga Castle. This choice would ultimately save Crona's life for the time being.
In the manga, before the battle at Baba Yaga Castle, Crona runs away from the DWMA and, with Medusa's influence, forgets about Maka as well as the others. While upset with Crona's turn to Medusa's side, Maka still believes somewhere inside Crona is her old friend, and determinedly tries to return them back to the DWMA's side. While her attempts fail at first, Crona finally is convinced by Maka when she risks her life to enter Asura and talk to Crona. Amazed she could be so selfless, risking her life for them and still caring about them after all they did, Crona decides to seal themselves inside the moon alongside Asura and the Black Blood, which allows Maka and the others to return to Death City safely. Maka later is shown to be deeply saddened and depressed about Crona's sealment inside the moon, spending a lot of time by herself when she returns home, thinking about it. Ultimately, she likely blames herself for what happened to them, as she was unable to convince them to leave Medusa before they were consumed by Asura. Still, after some encouragement from Soul, Maka expresses the belief that she will see Crona again and seems determined to do so in her last words of the manga.
Marie and Crona share a very close relationship, and Marie treats Crona in a very gentle, friendly fashion, resembling Maka's manner of treating them. Crona intensely enjoys this part of Marie and often becomes more happy and comforted when around her. Marie particularly likes Crona, regarding them as a "good kid" who is "so sweet and quiet". Marie acknowledges that Crona is an extremely gentle soul, and does whatever she can to help them no matter the situation. She also shows great enthusiasm in brightening Crona up, and enjoys seeing them happy, and making them feel better when something appears to be wrong with them. Marie has also stated that Crona is one of the focal points in which she enjoys teaching at the DWMA so much. When viewed side-by-side, Marie is the complete opposite of Medusa. In the manner of treating Crona, acting around Crona, and even in Crona's personality towards her. It is rational to believe that Crona may view Marie in a motherly way, and Marie doing the same. Marie has never exhibited as much kindness and friendliness as she does with Crona with any other individual in the series.
Despite Crona giving Soul a scar, Soul holds no grudge and tries to help out Crona. He appears to be friendly to Crona, mainly for Maka's sake, but he does show a little irritation such as when he threatened to kick Crona when they were being too frantic about the academy.
Soul seems able to see a bit of himself in Crona, which allows him to make some observation about Crona that their peers may not. In the penultimate chapter, Soul tells Crona to join in the song he has constructed from the world's souls, motivating the hesitant Crona by saying, "Just jump in there and worry about the details later. It's the same for me too." Soul makes this remark after Crona identifies as well how the two of them are similar: Crona tells Soul that he too understands how Maka gives them courage.
Tsubaki appears to be kind and gentle toward Crona. She is even friendly towards Ragnarok, offering him food and candy, despite his rather rude comments and disgusting mannerisms. Like Black☆Star, following Crona's defection to Medusa, Tsubaki struggles to convince Crona to rejoin their teachers and classmates at the DWMA, although whereas Black☆Star attempts to persuade Crona by beating them in combat, Tsubaki tries speaking to Crona.
Initially, upon finding Crona seizing all the souls on the Nidhögg, Kid was willing to kill Crona, as Medusa's child was on Death's kill list for being a potential Kishin. Following Crona's defection to the DWMA and Death removing the souls from Ragnarok, thereby mitigating the danger Crona and Ragnarok posed, Kid seemed to get along well with Crona, as Kid is seen not holding a grudge against them for what transpired at the Nidhögg.
When Medusa coerces Crona back to her side, and when Death ordered Spartoi to execute Crona, Kid was the only meister who did not object, as he believed it was necessary to follow his father's order. Nevertheless, as he clarified to Black☆Star, Kid also thought that even an order such as his father was not absolute—as he argued, no order is absolute—which led him to suggest, however cryptically, that he expected many of his peers were not going to follow through on Death's demand. Kid therefore argues that perhaps his father's order was actually a challenge to him and his peers to determine alternatives to killing Crona—but that finding Crona was of paramount importance.
Liz and Patty Thompson
When Patty first saw Crona, she couldn't tell whether they were either male or female, further pushing the idea that Crona's gender or biological sex has not been officially stated. They, like Kid, hold no grudge against them for what transpired at the Nidhögg. They are even seen helping Crona shop for clothes.
Medusa trained Crona in the hopes that they would one day rise to become the next Demon God. However, as it grows more and more apparent that Crona does not have the will to fulfill this task, Medusa spoke often of "throwing out" her unneeded child. Once Crona attends the DWMA, however, she forces Crona to be a spy and complete tasks for her.
In the anime, Medusa ultimately has no problem seemingly mortally wounding Crona, enticing Maka's fury. However, in the manga, she claims that Crona has been captured and that no parent wants their child to be sacrificed (though this turns out to be a ruse in order to manipulate Maka further).
Crona is the child of Medusa, Arachne's sister, which makes Arachne Crona's aunt. Like her sister, Arachne toys with Crona's emotions, possessing no love for the child and desiring only to exploit Crona to advance her own desires. Upon their first meeting, Arachne instantly recognized Crona as her sister's child, likely because her spiders were surveilling all parts of the world, including Medusa. As such, Arachne already knew Crona's abilities and the presence of Black Blood within the meister's body.
Arachne initially offered to recruit Crona to Arachnophobia, claiming that, even though Medusa showed Crona no love, as Crona's aunt, Arachne would. While shaking upon this meeting with an aunt, crying from the stress and anger at Arachne paralyzing Maka with her spider silk, Crona saw through her lie and attacked her. But as a powerful witch with 800 years of experience, Arachne defeated Crona with just attacks from her fan. Recognizing that Crona would not join Arachnophobia, Arachne had no qualms ordering Giriko to kill Crona and then Crona's friends Maka and Soul.