Like Peppermint Patty, Marcie is extremely fond of Charlie Brown. However, even though Marcie is more open about her feelings than Peppermint Patty, Charlie Brown seems oblivious to the feelings she has for him, and only thinks of her as a friend.
Marcie's opinion of Charlie Brown
Marcie suffers from unrequited love for Charlie Brown, but unlike her best friend, she is more open about these feelings. She often goes over to Charlie Brown, and asks him if he likes her. Unfortunately, Marcie is always rebuffed by Charlie Brown, as he never quite gets the message, even when Marcie outright tells him that she likes him.
Marcie seems to have fallen for Charlie Brown the first time she saw him, and has had a crush on him ever since. She shows her affection for Charlie Brown in many ways, for instance, telling him he is cute, asking him to hold her hand, calling him "Charles", sometimes kissing him, and even outright telling him she likes him and asking if he likes her back. However, despite all her attempts to get his attention, she never succeeds (though after one kiss he stood there wide-eyed in wonder with his hat hovering over his head), as Charlie Brown fails to recognize the feelings she has for him. Despite this, Marcie never gives up, and always tries to get him to notice her.
When Marcie is not trying to get Charlie Brown's attention, she acts very kindly towards Charlie Brown, and offers him advice, and makes him feel good.
In the strip from July 22, 1979 when Charlie Brown is hospitalized with a mysterious ailment, Marcie announces to Peppermint Patty that she thinks Charlie Brown is "real neat", and that she would marry him if he asked her to. Marcie's affection for Charlie Brown has been an occasional source of tension between her and Peppermint Patty.
Marcie's fondness for Charlie Brown does not completely blind her to other possible romances, and she developed a very affectionate relationship with Pierre during the events of Bon Voyage, Charlie Brown.
Charlie Brown's opinion of Marcie
Despite Marcie often telling Charlie Brown that she likes him, and sometimes Sally telling her brother that Marcie likes him, Charlie Brown is oblivious to Marcie's feeling towards him. This could be the result of Charlie Brown being unaccustomed to the idea of anyone liking him, as well as him being blinded to the affections of anyone who isn't the Little Red-Haired Girl (or perhaps Peggy Jean).
A running gag between them is that Marcie would ask Charlie Brown if he likes her, and Charlie Brown would reply "do I what?". On one occasion, when Charlie Brown got a letter from her in camp, he wondered why she always writes to him. Sally was able to figure out she likes him and Charlie Brown asked "Why would she like me?" to which Sally answered, "That's a mystery. You and the Bermuda Triangle."
Charlie Brown, does however like that she is nice to him, and thinks of her as a friend. And on some occasions, Charlie Brown is hinted to having feeling towards her. For example in You're the Greatest, Charlie Brown near the end after Marcie winks at Charlie Brown, he blushes, which can be interpreted as saying he likes her. He also blushes when she kisses him in There's No Time for Love, Charlie Brown. In a strip from 1989 while Marcie and Charlie Brown are in camp, he tells Peppermint Patty on the phone that Marcie is wearing a red swimsuit and looks really cute.