Ore no Shinyuu wa TS (Seitenkan) Taishitsu

Tsukushi Miokawa has a medical condition that turns him into a girl at times. His best friend, Kaoru, understands his situation and is always very helpful, like all best friends are. But what Tsukushi doesn't know is that, Kaoru loves how he looks when he turns into a girl. One day, the two friends take cover under a shelter to avoid the rain, but Tsukushi is in his girl form, soaked, with a boy's school uniform that isn't doing a good job of hiding the curves...
(Source: Irodori Comics)
Includes chapters 1.5 and 4.5.