Tsukiko is Tanpopo's first friend at Meio Academy. Though she appears rather innocent, Tsukiko originally is not very sweet at all, but she changes for the better toward the end of the manga. She has her heart set on marrying into the Kugyo family, and has her eyes on Koki.

Spoiler

Tsukiko does eventually get over her longing for Koki and lets Tanpopo have him all to herself, and afterwards appears to develop some feelings for Aoi, although this is never stated in canon.

Source: Wikipedia