Yasuke and Junko met on amicable terms in elementary school during the time when Yasuke's mother was struck with Alzheimer's. The disease caused her to lose recognition of her son, which resulted in him being neglected and abused. This made him severely depressed and introverted up until he and Junko met.
In the years following the death of Yasuke's mother, Junko became the person that Yasuke felt he could always lean on. She promised to always be there for him, and that she would never forget Yasuke even if she forgets everything else. For the first time in his life, Yasuke cried in front of another person, after making her promise she won't make fun of him, won't tell anyone and won't try to console him. He became dependent on her and ready to do anything for her, and they eventually became lovers.
Both of them later enrolled in Hope's Peak Academy, and after the school's tragedy, Yasuke reluctantly assisted Junko in her plans by creating her alternate identity, Ryōko Otonashi. He wanted to prevent her from ever remembering in order to protect her from herself and to protect other people. However, Junko had already planned ahead and had murders occur in the school. Despite not enjoying despair and killing like Junko did, Yasuke was willing to do unethical things for the sake of protecting her even if it caused him horrible guilt.
Yasuke eventually understands and admits to himself that Junko had only been using him. He became frustrated by her love for despair, and angered when he assumed that she never truly cared for him. He claims that he both loves and hates her, and eventually he decides he will kill her when she is still Ryōko. However, he ends up getting killed by Junko in a fit of despair, as he is one of the few people she cares about the most. He was stabbed in the side by Junko and afterwards, she kicked his body until it was a bloody unrecognizable mess. He died believing that Junko did not care about him at all, but in truth this was not the case. Junko did not view him as a mere servant or even an ally and truly did love him as he did her as killing him resulted in her sadness and grief for his death and let him die in despair for the sake of increasing her own despair, proving this with the words “... you were the most important person to me of all...” after his death.
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MatsuShima is not a very well known ship due to its origination from a side material, and it is not very well received either due to its dark ending. Despite this, it still has some supporters.