Karen is usually a very shy and timid girl who is even prone to passing out (called "The 'nyu's," for the syllable she utters before eventually passing out) when a situation gets too intense for her to handle. She frequently calls people by a title, such as calling Maiku, "Mr. Maiku", or "Maiku-San" in the Japanese version. Like Miina, she also develops feelings for Maiku, though she doesn't act on them for the same reasons. While looking at the picture of Maiku and the possible sibling she realizes that the picture wasn't taken at the house, but somewhere near the house. She explores the area around the house and finds an old shack with an open diary inside. Reading it she finally realizes the truth. She gets the nerve to ask Maiku on a date, violating the Love Alliance Rules. She wears the dress that Maiku bought for her. As the date comes to an end she reveals to Maiku that she loves him and is in love with him. When she realizes that Maiku is very shocked and stunned she finishes her sentence by saying that she loves him as a sibling would, due to the fact that she is Maiku's younger twin sister. In the last episode Karen finds out that Miina has left for the train station and quickly tells Maiku. Karen later follows after Maiku to see him and Miina kissing. She then jumps at them saying 'Ms. Miina you can't do it.' As they're all walking home together, Karen explains that she wants to be spoiled by her older brother for a little while longer and that Miina can't have him till then. Later she accepts their relationship and even encourages their love.