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- “It can't be you for more reasons than I can count.”
- — Jean to Mikasa, about her inheriting the Attack Titan.
Long before Jean even met Mikasa, he had drawn a picture of a pretty girl under his blanket in his room whilst his mother is yelling at him from downstairs. His “dream girl” looks exactly like Mikasa, from the eyes, smile, hair color, and up to the facial structure. His "dream girl" also appears to be smiling softly. This explains his shocked and flustered expression when he first met Mikasa, as she looks just like his artwork. Jean immediately develops a crush on Mikasa upon seeing her for the first time, causing him to blush and stutter. He tells her that she has beautiful hair, to which she replied with “thank you”. When Mikasa leaves, Jean follows her, wanting to say something more, but stops at his track when he sees her with Eren. He overhears their conversation and becomes jealous.
When Jean and the other cadets are stuck on the roof during the Battle for Trost, he remarks that it has been a dull life and laments over never having told ‘her’ how he felt.
Jean appears to be jealous of Eren whenever he is with Mikasa. When Mikasa stays with Eren while he is hurt and in bed, Jean is seen with a depressed face as he walks out the door. After Mikasa’s ribs are crushed by a titan, Jean becomes infuriated and stabs the titan in the eye, freeing Mikasa and saving her life. He later points out how big the scar on her cheek is. This scar was acquired due to Mikasa saving Eren.
When Jean is disarmed and about to be shot at the beginning of the Uprising Arc, Mikasa shouts his name and rushes to save him. However, it is Armin who ends up saving him. Later on in the manga, Mikasa is shown treating Jean’s wounds during the Battle of Shiganshina.
One day, after they've finished working on a railroad track, Eren ponders who to choose as his inheritor of the Attack Titan after he dies in five years; Mikasa willingly volunteers to take upon Eren’s titan after he passes away in four years, but Jean is quick to interrupt her. Jean says that she cannot be the next titan shifter as she’s half-Eastern and an Ackerman. He proposes that no one knows what will happen to her if the titan powers are passed on to her as Ackermans, in general, are a mystery. Jean mostly likely said this because of his feelings for her, and he wants her to live a long life. If she were to become a titan, her lifespan would shorten to only about an extra thirteen years.
After Jean hears about Eren’s mistreatment of Mikasa, he grows angry and demands Mikasa to tell them how Eren has hurt her. Mikasa tells him that she’s fine, but he is insistent, which becomes intense and results in Connie telling him to cut it out. This shows how much Jean still truly cares about Mikasa, for a moment letting his usual a facade of indifference slip. When the two are stuck with the Jaegerists, they are contacted by Hange who tells them about the plan they worked out to stop the Rumbling. Jean, for some time, has to pretend to be one of them, but it's apparent they both seem to despise the group. When Mikasa receives a secret signal from Jean, she leaves the military headquarters for Shiganshina.
After learning about Eren’s plan to destroy the rest of the world, Jean is shown to be dreaming of a ‘happy life’ with a wife and a child, he even thinks about living to see his grandchildren. Jean’s wife in his dream has medium length black hair and a scar on her right cheekbone, like the one that Mikasa received while saving Eren years ago — overall, the woman seems to resemble Mikasa greatly. She appears to be holding a baby in her arms, which could signify that after all these years Jean still has feelings for Mikasa and wishes to build a family with her in the near future if they are to survive. Jean is also the one who directly reveals to Mikasa about how killing Eren would be the only way to ultimately stop the Rumbling. He tells this with an extremely pained expression, as he is obviously aware of Mikasa’s feelings towards Eren.
The two survive the war, and are shown to be alive three years later. Mikasa lives on Paradis Island, while Jean, who is now a peace ambassador, stays on the continent. Jean, along with his friends, is seen sailing to Paradis Island. As Jean reprimands Reiner for still thinking about Historia, a married woman, Pieck teases Jean about taking great care of his looks as if he were getting ready for something or someone. Jean is shown styling his hair and not taking his eyes away from the mirror — although he says he just wants to look good for the girls who read history books. On Paradis, as Mikasa visits Eren's grave, she tells him their friends are coming to visit soon.
- Jean – “H-hey, you! Ah... Er... I just haven't seen a face like yours before, so I just... Sorry, you have really beautiful black hair.”
- Mikasa – “Thanks.”
- — Jean and Mikasa's first meeting, Chapter 15
- Jean – “Eren, I heard you tried to kill Mikasa when you turned into a Titan. What the hell was that about?”
- Mikasa – “Wrong. Eren was trying to swat a fly.”
- Jean – “I'm not asking you!”
- — Jean confronts Eren for hurting Mikasa, Chapter 22
- Armin – “It might be about time they (the Titans) decide to go for a walk all at once.”
- Jean – “Was that a joke right there, Armin? Man, you're lame! That was great!”
- Mikasa – “Jean. Quiet.”
- Jean – “All right! S-sorry.”
- — Chapter 34
- Jean – “He (Eren) hurt Mikasa? How?”
- Mikasa – “Stop it. It's... fine.”
- Jean – “No, it's not. Tell me how he hurt her.”
- — Jean shows concern to Mikasa Chapter 116
After the Rumbling, Jean is seen styling his hair. It has been three years since the rumbling, and Jean and the others are going to Paradis. It is very likely that he may be styling his hair in order to impress Mikasa, as he has not seen her in 3 years. A number of fans also believe that Mikasa might fall in love after Eren’s death or move on with someone else eventually. This could potentially be Jean given the fact that they have spent four years in close proximity with each other, in addition to his unwavering feelings for her.
The rise to the ship’s popularity was given by Jean’s drawing, often referred to as “Jean’s Dream Girl” in the fandom, implying that Mikasa is the girl from the drawing, even though the two hadn’t met yet. In addition to this, Jean’s deep feelings towards Mikasa throughout the series has made it one of the most popular ships in the franchise. The ship is currently Mikasa's third most popular and Jean's fourth on Archive of Our Own, and it has almost a thousand works under the “Mikasa Ackerman/Jean Kirstein” tag.
Sometimes in recent fan works, Mikasa can be seen more dominant, or using the word "girlboss" from fans, meanwhile Jean is much more submissive and fans use the word "malewife" to define him.
- In the Attack on Titan AU Smartpass: Bertolt’s Goodnight Sweet Dreams, when the boys talk about dreams and Bertolt’s awkward sleeping position, Connie cuts in and says, “Speaking of which, Jean also had one… about Mikasa…” This makes Jean flustered and embarrassed, to which he says, “I-Idiot! Shut up!” 
- In another AU Smartpass called Late Night Stories Vol. 05 Reiner Braun A Story of Absent-Mindedness, Jean gets jealous and angry at Reiner since two female training corps were admiring his body. In response to this, Reiner reassures him by saying, “Oh! Well, Mikasa wasn’t there, you know!” 
- The AoT Guidebook notes that as Jean has feelings for Mikasa, he felt a little "chilled" when he noticed her special attitude towards Eren. 
- Jean's Japanese VA is a JeanKasa shipper.
- In one of his tweets he questioned if Mikasa let Jean wrap the scarf around her, or Jean wrapped the scarf around Mikasa alone.
- Attack on Titan OVA: A Sudden Visitor, Episode 3.25.
- Attack on Titan AU Smartpass: Good Night, Sweet Dreams, Dear (Bertolt)
- Attack on Titan AU Smartpass: 'Round About Midnight (Reiner)
- Attack on Titan Guidebook: INSIDE & OUTSIDE, Chapter 3, p. 116.