Calvin-Susie Derkins relations is the belief that Calvin and Susie Derkins are secretly in love with each other. There are numerous pieces of evidence located within the comic strip to support this.
Calvin once made Susie a valentine and flowers (albeit a hate-mail valentine and dead flowers, but it showed he cared).
In that same story arc, Calvin was left thinking "She noticed! She likes me!".
In one Sunday strip, Calvin and Susie exchange insults, and Calvin says "It's shameless the way we flirt".
Calvin's good side began writing romantic love poems to Susie and offering to carry her books for her and sent her flowers. As it is a projection of Calvin's good side, and therefore, is half of him, this practically confirms Calvin's love for Susie.
In one strip when Hobbes was stolen by a big dog, when Calvin found out that Susie had Hobbes, he kissed her hand, but Hobbes stole her cookies.
Calvin sits with Susie during lunch.
When Calvin calls Susie a "booger-brain" and makes her cry, he genuinely feels bad and is upset when she initially ignores his attempt to apologize.
When she received the hate-mail valentine and dead flowers from Calvin, she thought "A valentine and flowers! He likes me!"
In the story arc with the last meeting of G.R.O.S.S., Susie raced to the hall closet, believing Calvin to be trapped in it.
When Calvin calls asking about homework, she tries to make him say he "missed the melodious sound of her voice".
She invited Calvin to her birthday party.
Although she is grossed out by Calvin's eating habits, she still sits with him during lunch.
A popular fan theory is that Calvin and Susie end up getting married as adults and have children of their own.