Darrin has low intelligence, but he is very friendly, and is shown to care for others, perhaps the most out of all the "Axmen."
Sue Heck (2013; 2014-2015)
He takes Sue to the prom in The Clover but developes feelings for her in Valentine's Day IV. In this episode Matt breaks up with Sue; and when Darrin tells her the news, he also says that he will be her date to the prom so that she won't be alone. At the end, she thanks him being nice for the night, but he reveals he didn't go to the dance with her just to be nice, and they kiss.
Sue and Darrin become a couple but they also become the "Yoko and John" of Axl's band. Axl gets mad at them and he threatens Darrin saying that if he and Sue will stay a couple, he will break up his friendship with him. Sue and Darrin have to break up their relationship.
Some episodes later in Season 5 when he breaks up with Angel, he and Sue realize they are both single. That doesn't make any difference until the new prom. In The Walk, in fact, he invites her to the prom and she accepts, with him being her fifth invite. When he finds out he decides he won't go the prom with her anymore, but when Axl tells him Sue went to the prom alone because she declined the other invites because she is still in love with him, he reaches her at the prom and they kiss.
They announce they are dating again some days later, but Mike doesn't accept Darrin because he thinks he is too immature and old for Sue.
In Season 6 he gives her a special necklace to proclaim his love but when Sue finds out he also gave it to his ex-girlfriend they start a fight but immediately make up. Sue also tries to help Darrin be approved by Mike, but he makes things go even worse with Mike hating him.
In Valentine's Day VI he organizes a scavenger hunt for Sue that ends at Darrin's house with him proposing to her, with also a ring. In The Answer Sue is panicked to marry him and can't find the words to tell him. When Mike angrily says he will kill Darrin, she intervenes and tells him she will break up the engagement. After several failed attempts, Sue finally tells Darrin she has many other things she wants to do before getting married, and breaks off their engagement and relationship.
Angel is Darrin's new girlfriend after he breaks up with Sue at the end of Season 4. Angel is a bad girlfriend, that wants everything to be the way she wants and acts like Darrin is her slave. When Darrin rebels to his friends in Sleepless in Orson because they act like Darrin is a third wheel, he also decides to break up with Angel. He gets single only to have another relationship with Sue.