Both Mikan and Seiko were students of Hope's Peak Academy. Since they attended different classes, with Seiko being in Class 76th, and Mikan being in Class 77-B, they didn't interact much in canon. Although they were just students, they both secured faculty-like positions as the school's official nurse and pharmacist respectively. With the two being in two different classes, it is unknown if they had ever gotten the chance to interact with each other during their time at Hope's Peak, especially due to Seiko being expelled along with her friends and classmates, Ruruka Ando and Sonosuke Izayoi. It is known that Mikan was once drugged with aphrodisiacs that Seiko concocted, when Hiyoko Saionji decided to pull a prank on her class.
The two girls have canonically encountered each other at some point, after Mikan and the rest of her class became the Remnants of Despair, and Seiko and her ex-classmates joined the Future Foundation to combat despair. During a flashback sequence in the first episode of Future Arc, Seiko was shown stopping Mikan from jabbing a syringe into her patient's eye, quickly smacking it out of her hand.
While they did not directly work together, both girls played a role in saving the life of Kyoko Kirigiri. In Hope Arc, it was discovered that Seiko had developed an antagonist called "Cure W" to counteract the effects of the poison-injecting bracelets that were used in the Final Killing Game. At some point, Kyoko took the drug after stumbling upon Seiko's corpse, which allowed Mikan to revive her from her near-death state.
The ship sailed due to both characters sharing many similarities. Both are self-conscious and soft in nature, and are shown to have extensive medical knowledge in their respective fields. They are both also known to be victims of bullying from Hiyoko and Ruruka respectively, even though they are supposed to be best friends. This makes them two of the most sympathized characters in the fandom, which compelled some fans to ship them, believing they would be able to relate to each other.
In spite of their numerous similarities, the ship is not overly popular in the fandom, largely because both characters are involved in more popular ships, namely Band Aid for Mikan, and Toxicandy for Seiko. However, most fans agree that the latter ship is unhealthy, thus giving Seikan a slightly more positive reputation.
The lack of popularity also comes from the fact that the two only have one brief interaction in canon. Danganronpa 3 receiving mixed feedback and being overlooked by many fans has also contributed to this. As such, this ship doesn't have that many fan fictions or fan arts dedicated to it, although there's still a fair amount of content for such a minor ship. Nonetheless, in spite of all this, it remains as one of the more popular shipping choices for Seiko.
- Both characters are blood type A.
- Both girls have a medical-themed Ultimate talent.