Megumi is Haruka's little sister. She's young, restless, and a little naive, but has enough energy to light up any room she's in. At the tender of age of 7, she was separated from her parents and moved in with her uncle Rikudoh along with Haruka (then 22 years old) who was assigned to work for TERRA. Megumi has always looked up to Haruka, but as she gets older, and this is evident at the beginning of the series, she actually holds a little remorse and some jealousy towards her big sister. Though beautiful herself, she feels inferior to Haruka.
Like all girls her age (14), she's bound to have several crushes. Throughout the series she has two major crushes. The first and biggest one is with Yagumo Souichi, to whom she even gives a present during Christmas --even in a similar way when Haruka gave Ayato a pair of mittens. However, she later finds out that Souichi and her best friend Kim Hotal have been seeing each other secretly. Though it broke her heart to find out the way she did (by accident), she eventually decides to move on.
It wasn't until after this incident that she starts to notice Ayato and later develops feelings for him, probably more so than a simple crush as she could easily relate with him in many ways (I think they had good chemistry). However, her heart gets broken yet again when she finds out that Ayato is actually Haruka's old boyfriend (first love even) when they were in Tokyo before the Mulian invasion and the change in time due to the Jupiter barrier. She found this out when collecting a few of Haruka's belongings and finds the picture of Haruka together with Ayato at 14 years of age. On episode 24 she decides to give Ayato up to Haruka by yelling it before leaving for safety.
Though Megumi doesn't take on any big roles in the story, she's an important part of the drama/romance in RahXephon.