Sheets no Sukima

Between the Sheets features the unusual friendship from the nearly inseparable Saki and Minako. Minako carries a fervent torch for the other girl - a passion of which Saki is initially ignorant. But when Minako sleeps with Saki's boyfriend Ken (just to see what Saki sees in him), the true nature of Minako's obsession begins to reveal itself ... provoking an understandably strong reaction from Saki.
(Source: Tokyopop)